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THE AM BULLETIN BOARD => QSO => Topic started by: K1JJ on August 31, 2020, 01:36:40 PM



Title: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on August 31, 2020, 01:36:40 PM
I'm in the thinking stage for a 20-35 watt carrier Pissweaker AM tube rig that is plate modulated.  Self contained with PS and VFO preferred. Chinese DDS VFO OK.   Super high fidelity mandatory.

This will probably be my last HB rig for a while.  I want to take my time and do it up right with newer parts and metalwork.  There is a lower power gap now with my smallest AM rig at about 125 watts carrier output.

Please post your thoughts and schematics. What smaller rig have you always wanted to build?  I am going to limit it to plate modulated using a conventional class C final and class B p-p modulators.  A minimum of iron; no audio driver xfmrs - just the mod xfmr and mod reactor (if needed) are preferred.  Power supply iron is OK and expected.

What little rig have you always wanted to build? What advantages, reasons to go this route?  Also looking for mod iron to match the project.

T


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: Opcom on August 31, 2020, 02:58:09 PM
Always a tube type "Part 15 AM broadcaster" in a rack mount chassis, with panel meters like any transmitter. Plate modulated and class C final amp. DDS would be OK or a crystal. Sort of a 100mW-input-rule set disguised as something of the power level you are considering. A stand-in for the missing AM Oldies station -gone several years now and I have a few nice old console radios on which I liked to listen to big band music. I could buy a high quality modern solid state synthesized unit for $400, but I'd like to try building it first. This is not near the top of priorities in my life though.


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: Tom WA3KLR on August 31, 2020, 07:47:59 PM
Hi Tom,

Didnít you inspire a bunch of 5 Watt piss-weak rigs about 10 -15 years ago and you had one?

Anyway, the DX-60ís with the WA1QIX? screen series modulator mod is very good.  Thatís my recommendation- a screen-modulated 6146.  You can build one or start with a DX-60.  I donít know if they have other weaknesses.  You can have audio response down to dc and since no modulation transformer, could implement an rf feedback audio if desired with no problem? But the feedback is not necessary but doable in this case.

GL OM.


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on August 31, 2020, 09:20:51 PM
Hi Tom,

Didnít you inspire a bunch of 5 Watt piss-weak rigs about 10 -15 years ago and you had one?

Anyway, the DX-60ís with the WA1QIX? screen series modulator mod is very good.  Thatís my recommendation- a screen-modulated 6146.  You can build one or start with a DX-60.  I donít know if they have other weaknesses.  You can have audio response down to dc and since no modulation transformer, could implement an rf feedback audio if desired with no problem? But the feedback is not necessary but doable in this case.

GL OM.


Interesting idea, Tom...

Frank/GFZ also suggested a screen modulated rig; a pair of 6146s screen modulated by a 11N90 MOSFET. Probably good for 25 watts out and audio DC to light. Though I'd like to stay with all tubes like the transformerless DX-60 circuit for example.

Yes, the 1994 Pissweakers net had 25 homebrew rigs check into the first net using 5-10 watt rigs. That started many guys homebrewing bigger rigs later on. I remember Rich/K1ETP went from his first plate modulated project, a 10 watt cakepan 6AQ5 -  to later building an 813 rig.  I still have my original 5 watt 6AQ5 series modulated rig, but want to do something else.

Steve/QIX's screen modulated circuit is quite appealing, no doubt.

T


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: WU2D on August 31, 2020, 10:15:34 PM
Yup that was Frank.There were a bunch of wacko rigs on. Series and Screen modulated 6AQ5's, 50C5's and so on.  I did up a suppressor grid modulated 6AG7 and then upgraded to an 837 Oscillator. 7W or so out. Just a Carbon mic and a tube.


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on August 31, 2020, 10:24:22 PM
Yup that was Frank. There were a bunch of wacko rigs on. Series and Screen modulated 6AQ5's, 50C5's and so on.  I did up a suppressor grid modulated 6AG7 and then upgraded to an 837 Oscillator. 7W or so out. Just a Carbon mic and a tube.

Is your avatar picture the PW rig you described?

Yes, I remember you running that rig. I have a folder of schematics the guys sent me back then. Maybe yours is in there too.

I have been getting messages saying the transformer-less screen modulated or series modulation approach is the way to go for a PW rig.  Just generating ideas right now. I've run both before and have had good luck.  As long as the series tube is a big mofo compared to the final... and the screen modulated rig is carefully tuned up with lots of loading, they are hard to beat. 20-30 watts output is not too much of a strain.  

The more I think about it, I already have three plate modulated rigs and no other types. I should build a PW series or screen modulated rig.

I have been hearing that a series modulated rig is less stable for RF than a screen modulated rig or plate modulated. What is it about instability with a series modulated rig?  The ones I built had some minor problems that I was able to correct...  These were the 6AQ5 series modulated and PDM tube rigs.

T


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: KD1SH on September 01, 2020, 12:09:38 PM
  I second that notion: the QIX screen mod is fantastic, and I'm sure the concept could be easily applied to a HB rig. Matter of fact, I plan to do just that: maybe two 6146's instead of one - just to drive my amp a bit harder, or maybe a bit more voltage on the plate of a single - a better power supply than the stock DX-60; some better metering; stuff like that.
  There's little not to like: clean audio from 10 hz to 20 khz, no mod iron, and enough output to drive most amps up to a decent carrier. Audio reports have been better than I could have hoped for, especially given that I'm not blessed with an outstanding "broadcast voice", and watching the output on my REA mod monitor is almost a religious experience. Negative peaks limited, and those positive peaks!
  Efficiency becomes an issue, of course, if we're talking big rigs, but for PW/exciter class rigs, Steve's circuit has gotten me hooked on screen modulation.


************************

Anyway, the DX-60ís with the WA1QIX? screen series modulator mod is very good.  Thatís my recommendation- a screen-modulated 6146.  You can build one or start with a DX-60.  I donít know if they have other weaknesses.  You can have audio response down to dc and since no modulation transformer, could implement an rf feedback audio if desired with no problem? But the feedback is not necessary but doable in this case.

GL OM.



Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on September 01, 2020, 12:42:31 PM
http://amfone.net/Amforum/index.php?topic=26603.0


Here's the QIX screen modulated circuit in case someone is interested.  I am thinking about it but with a bigger tube to make 25-30  watts output AM carrier.

If I do, I'll draw up a complete schematic and post it here for all to critique.  Maybe a self contained 4D32  (or two?)  screen modulated  -    using a DDS VFO would do the trick.  

I like the idea of using a negative peak limiter and really pile on the audio. I normally don't do this with the bigger rigs, but a PW rig could really play well with the extra audio.

Steve said: "The modulator features a negative peak limiter, set to approximately 93% negative.  When properly adjusted, the DX-60 with these modifications will modulate more than 150% positive and will give a really fine reproduction of the input signal.  Frequency response is quite flat from 5hZ to over 10kHz."

Here's an actual recording of the screen modulated modified rig:

http://amfone.net/Amforum/index.php?topic=26603.0


Hmmm... I'm getting more interested...


T


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on September 01, 2020, 01:17:05 PM
N2DTS wrote:
"I just wanted to update it in case anyone else wanted to try the circuit, which works great within its limitations.

And I think some tubes just will not sound clean no matter how you modulate the screen.
I lump the 813 and the 4D32 into that category, the 6146 and the 4x150 seem good.
Maybe its tubes that use lower screen voltages that work best, and with this circuit, ones that do not pull much current on peaks and some tubes likely do not track plate current with screen voltage well.

From what I see, it seems like the output impedance is whatever the cathode resistor is?
In this case, 39k ohms?

In my tests, the 4D32 was flat topping easy unless run at very low power outputs (5 watts).
The solid state modulator worked fine with the 4D32's, even 3 of them at once.

I am sure Steve could design a simple mosfet circuit with a real low impedance that took little drive and was good for all sorts of voltage and current.
There are some amazing devices out there these days."



So maybe a MOSFET screen modulator would be a better choice for a 4D32.   I have a MOSFET circuit that Frank/GFZ designed... I may take a closer look at that.

T


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: KD1SH on September 01, 2020, 02:20:05 PM
My audio chain is slightly asymmetrical in the positive direction, so I'm hitting 130% peaks to even 150% without pounding into the negative limiter; just tickling it a bit. Nice thing is that, in the absence of mod-iron, you can't hurt the thing. Aside from common courtesy issues, of course, you can pretty much lay into it and let the peaks fall where they may. As always, though - maybe even more so than with plate modulation - a scope or mod monitor is crucial, especially when loading. You can go from silky smooth audio to monkey-snot with a small turn of that loading knob.

http://amfone.net/Amforum/index.php?topic=26603.0


Here's the QIX screen modulated circuit in case someone is interested.  I am thinking about it but with a bigger tube to make 25-30  watts output AM carrier.

If I do, I'll draw up a complete schematic and post it here for all to critique.  Maybe a self contained 4D32  (or two?)  screen modulated  -    using a DDS VFO would do the trick.  

I like the idea of using a negative peak limiter and really pile on the audio. I normally don't do this with the bigger rigs, but a PW rig could really play well with the extra audio.

Steve said: "The modulator features a negative peak limiter, set to approximately 93% negative.  When properly adjusted, the DX-60 with these modifications will modulate more than 150% positive and will give a really fine reproduction of the input signal.  Frequency response is quite flat from 5hZ to over 10kHz."

Here's an actual recording of the screen modulated modified rig:

http://amfone.net/Amforum/index.php?topic=26603.0


Hmmm... I'm getting more interested...


T


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on September 01, 2020, 04:05:09 PM
My audio chain is slightly asymmetrical in the positive direction, so I'm hitting 130% peaks to even 150% without pounding into the negative limiter; just tickling it a bit. Nice thing is that, in the absence of mod-iron, you can't hurt the thing. Aside from common courtesy issues, of course, you can pretty much lay into it and let the peaks fall where they may. As always, though - maybe even more so than with plate modulation - a scope or mod monitor is crucial, especially when loading. You can go from silky smooth audio to monkey-snot with a small turn of that loading knob.

A pair of 6146s might be another possibility cuz the tube circuit you guys are using is proven.  How much carrier out do you get with one 6146 at 150% positive peaks?  Yes, the loading is sensitive.

What maximum HV would you suggest?   25-30 watts out would be FB, but may be axing too much.  Maybe three 6146s.   I wonder if 1200 volts is about right... like a 600V plate modulated rig in a Ranger, but screen modulated.  400V dead carrier and 1200V peak under modulation.

T


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: KD1SH on September 01, 2020, 05:09:51 PM
  With my DX-60's single 6146 I can get a bit over 15 watts carrier, but when loaded for proper modulation the max carrier is about 12 watts. That's with the DX-60's stock power supply, which delivers 600 volts. The 6146B, as I remember, can handle 750 volts intermittently.
  So, I think I'll try pine-boarding something with a single 6146B at 750 volts, to start, since I'm only looking to get 15 to 20 watts for driving my amp. I'll make a few changes over the stock DX-60: a cleaner power supply (the DX-60's B+ winds up on the plate of the modulator/cathode follower as well as the 6146, so any ripple winds up in the audio); a nice clean regulated grid bias supply instead of grid-leak bias; better metering; and I think I'll set the bias to push the 6146 a bit further into class C than the DX-60 does now.

My audio chain is slightly asymmetrical in the positive direction, so I'm hitting 130% peaks to even 150% without pounding into the negative limiter; just tickling it a bit. Nice thing is that, in the absence of mod-iron, you can't hurt the thing. Aside from common courtesy issues, of course, you can pretty much lay into it and let the peaks fall where they may. As always, though - maybe even more so than with plate modulation - a scope or mod monitor is crucial, especially when loading. You can go from silky smooth audio to monkey-snot with a small turn of that loading knob.

A pair of 6146s might be another possibility cuz the tube circuit you guys are using is proven.  How much carrier out do you get with one 6146 at 150% positive peaks?  Yes, the loading is sensitive.

What maximum HV would you suggest?   25-30 watts out would be FB, but may be axing too much.  Maybe three 6146s.   I wonder if 1200 volts is about right... like a 600V plate modulated rig in a Ranger, but screen modulated.  400V dead carrier and 1200V peak under modulation.

T



Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: WB3JOK on September 01, 2020, 10:43:50 PM
How homebrewed do you want it - starting from a bare chassis? :)

I have 80 and 40 meter "ARC-5" transmitters and built a screen modulator. There's a thread on AMfone a few years back (may be on the tech forum) with schematic, pics etc. The typical '60's speech amp circuit from the ARRL handbook, and a 6AQ5 for the modulator tube. The rig only puts out about 10W carrier but it looks good on the scope.
As already mentioned, loading adjustment is critical for a screen mod, and I'm glad I didn't put it on the air without a scope! Especially with the 4x4-125A amp  8)


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: KD6VXI on September 02, 2020, 08:37:32 AM
The 6l6 series of tubes may be good in this spot, too.

A Trio of them would look cool as well as provide the power your looking for I think.  A quad may be better.

At 75 meters, the KT88 works fine business.  Heck, I've //gotten// a KT88 working on ten meters, but just barely.

Just a thought, there is life other than the 6146, if you want to be different 🤔

--Shane
KD6VXI


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K8DI on September 02, 2020, 10:39:06 AM
I've seen a lot of variations on ways to do 25w (more or less) suggested here, many of which will give a great sounding signal, I'm sure.  But I wonder if maybe you should be asking yourself, do I want something unique, something complicated, something maybe less stable, and so on, or do I want a back-to-basics simple rig?  The ideas of hifi audio and series mod and solid state all are great, don't get me wrong, but will you be happiest with that, or a simple little plate mod rig? A single 807 pentode can do 40+ watts, you like to run them up and turn them down in general use, if you want a triode, an 809, a little less common these days than an 807, good for even a little more maximum output. Find a 75 watt class mod transformer, build up a fully differential audio amp with feedback, and optimize the piss out of it, and you'll have a low parts count, classic, basic but great, 25 watter. But only if that's what you want.....either way, I'm watching, looking forward to whatever you share...

Ed



Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: KD1SH on September 02, 2020, 01:45:28 PM
  "Complicated", I think, is in the proverbial eye of the beholder, and I suspect that many of us are here - and into AM in general - because we like being technically creative. Tom is certainly one of the most ambitious builders around; he's probably built just about every sort of rig there is to build, and I assume he does it because he likes the technical challenge. It doesn't appear that he's gotten bored yet. Me, if I wanted perfect reliability and stability, I'd run a rice-box, but I'd soon be bored to death.
  To me, screen modulation is simpler than conventional plate modulation, and good mod-iron isn't getting any easier to find. Series mod can get complicated largely because you've got to isolate parts of your circuit, with high potentials floating around, otherwise it's pretty straightforward (check out the WB9ECK series mod rig, http://www.amwindow.org/tech/htm/wb9eckseriesmod.htm).

I've seen a lot of variations on ways to do 25w (more or less) suggested here, many of which will give a great sounding signal, I'm sure.  But I wonder if maybe you should be asking yourself, do I want something unique, something complicated, something maybe less stable, and so on, or do I want a back-to-basics simple rig?  The ideas of hifi audio and series mod and solid state all are great, don't get me wrong, but will you be happiest with that, or a simple little plate mod rig? A single 807 pentode can do 40+ watts, you like to run them up and turn them down in general use, if you want a triode, an 809, a little less common these days than an 807, good for even a little more maximum output. Find a 75 watt class mod transformer, build up a fully differential audio amp with feedback, and optimize the piss out of it, and you'll have a low parts count, classic, basic but great, 25 watter. But only if that's what you want.....either way, I'm watching, looking forward to whatever you share...

Ed




Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on September 02, 2020, 06:23:38 PM
Thanks for the thoughts, guys.

Yes, there is life after 6146s.  They are a boring tube for sure.  But they are possibly the best performing tube in its class.

Building rigs and homebrewing: I like looking at and turning the knobs after they work well. I use slide-on enclosures so I can see the insides in seconds. Three rigs use Plexiglas covers for easy viewing.  I like looking at the work and the tubes themselves. Flipping on the switches and everything works right is fun.  It's fun just looking at the stuff in the open. In contrast, a rig hidden in a rack never gets seen for long periods of time.

I get no thrill looking at commercial gear like my FT-1000D or SDR receiver. But they are necessary and near impossible to homebrew.  The buildable homebrew gear is where it's at Daddio.

I've owned my share of boatanchors... probably every one made at one time. The transmitters were always breaking down. Rangers, Valiants, etc., gave me a lot of trouble. But my homebrew stuff, after working and tested well, rarely craps out. And if it ever does, I can fix it easily cuz I'm the one who designed and built it.  In contrast, when the FT-1000D craps out I feel panic coming on and 1/2 the time I can fix it and 1/2 the time I send it out to the Yaesu shop.

There is a slow rate of workmanship improvement with homebrew rigs that you can see comparing early rigs to later ones. We simply get better at it.

My first rig project was the 811A linear in the 1964 Handbook. I was 12 years old.  I didn't have any parts yet so started the chassis work. I used my metal Gilbert chemistry set as the chassis and a hammer and nail to cut the hole for the tube socket. The chassis collapsed from the pounding and that was the end of that project.  The next one was the swr bridge in the same handbook. I never got it working.  I then tried a one tube radio receiver. That never worked either. It was a rough start with no mentors.

The planning stage is probably the most fun...   Here's a strange looking tube that is plentiful on eBay; the 815 dual beam powered tetrode.  Two tubes strapped in parallel will do about 25 watts screen modulated. I am considering it with the other ideas.

https://tubedata.altanatubes.com.br/sheets/049/8/815.pdf

A spacesuit robot looking tube.

**BTW, a few months back I contacted Bob/ECK about his series modulated rig.  He said it later became unstable. He changed the series modulators to different tubes and never got it working to his satisfaction. I was thinking of something like that.    My 5 watt  6AQ5 series modulated rig could be scaled up to 30 watts.... that still works FB. As already said, the HV isolation and big heat from the series modulator is a factor.

http://www.amwindow.org/tech/htm/series.htm

T


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on September 02, 2020, 10:58:54 PM
I'm down to two designs:

I'm thinking I may build up the QIX screen modulated audio circuit. It is a proven design and if I get stuck I can always axe Steve.  I can build the audio and power supplies and leave the RF section clear for now until I decide on a 20-30 watt final(s). The equivalent of three 6146s putting out 30 watts carrier would be FB.

The second choice is the series modulated scaled up version of my 6AQ5 rig without the self-excited xtal circuit.  Either one will be very clean and have huge audio capability.

I'll start gathering general parts and decide on a chassis.  This rig should be lighter to carry around than the last three I've built.

Oh, another thing about the fun of homebrew rigs is hanging out on the air with a bunch of like-minded builders. Yesterday I was on 3885 trying out Baby Blue for the first time in a live QSO. On there with me was Frank/GFZ testing his new audio processor with his highly modified Valiant. Also was Rich/ETP another builder of a nice 813 rig. And then there was Bob/KBW talking about his class E rig. And Clark, another class E builder. The Tron checked in later on with his new pair of 813s in linear and was able to give me an audio report on the new rig. He said "don't touch a thang!"  

Everyone had a chance to talk/brag about their particular rig. Nowhere else in the "normal" world can you go and people are willing to listen and talk about their rigs.... :-) How many family members or regular non-ham friends would care about your new 813 rig?  Having a posse to get on with is so important.   The same benefit goes with using AMfone.net.

T


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: KD6VXI on September 02, 2020, 11:55:44 PM
I'm liking the KT88 more and more, as I look at it.  A pair, scream modulated would be a trifecta of 6146s.....

Or the 6550.

I like tubes that are still in production.

--Shane
KD6VXI


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: KD1SH on September 03, 2020, 08:43:51 AM
  That's too bad; that rig, along with a post by Steve, QIX, about series mod, inspired me to start building my own PW series mod rig (which hasn't yet gotten off the pine-board, mostly because I got distracted). I wonder what went wrong. Unstable in the RF section, or audio? Since he changed the modulator tubes, I'm suspecting audio. Series mod is cool, but I suspect there are a few more gremlins lurking there than in a screen mod rig.


**BTW, a few months back I contacted Bob/ECK about his series modulated rig.  He said it later became unstable. He changed the series modulators to different tubes and never got it working to his satisfaction. I was thinking of something like that.    My 5 watt  6AQ5 series modulated rig could be scaled up to 30 watts.... that still works FB. As already said, the HV isolation and big heat from the series modulator is a factor.

http://www.amwindow.org/tech/htm/series.htm

T


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: KD1SH on September 03, 2020, 10:13:23 AM
A third choice, if you want to be bleeding-edge unique: heater modulation. Good for some really, really, low audio freqs!  ;D

I'm down to two designs:

I'm thinking I may build up the QIX screen modulated audio circuit. It is a proven design and if I get stuck I can always axe Steve.  I can build the audio and power supplies and leave the RF section clear for now until I decide on a 20-30 watt final(s). The equivalent of three 6146s putting out 30 watts carrier would be FB.

The second choice is the series modulated scaled up version of my 6AQ5 rig.  Either one will be very clean and have huge audio capability.



T



Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: w8khk on September 03, 2020, 12:29:55 PM
I'm down to two designs:

I'm thinking I may build up the QIX screen modulated audio circuit. It is a proven design and if I get stuck I can always axe Steve.  I can build the audio and power supplies and leave the RF section clear for now until I decide on a 20-30 watt final(s). The equivalent of three 6146s putting out 30 watts carrier would be FB.

The second choice is the series modulated scaled up version of my 6AQ5 rig without the self-excited xtal circuit.  Either one will be very clean and have huge audio capability.  snip.....

Tom, whether you go screen modulated or series modulated, the overall dissipation power budget is simply moved between the RF final and the series modulator.  There are lots of screen mod rigs out there, but very few series mods.  Personally, I would trade the tuning and loading issues of a screen mod rig for the work in engineering a very linear series mod rig, and then operating and tuning is identical to the ease of a plate modulated rig.

If you place the series tube above the final, the only thing you need to isolate is the tube socket, and use a filament transformer with very good secondary insulation with high breakdown voltage.  Here are some ideas I might try:

Find an old HP13, SB-400, SB-401 junker and grab the power transformer.  That and another filament transformer is all the iron you need.  Using 1N5348 diodes and very inexpensive and small axial or radial caps under the chassis, and the dual-voltage power supply is finished.  250 or 300 volts for the exciter, ample bias, and 350/700 volts for the final.  While an RCA 808 would be pretty and retro, and the 6146 is sameo-sameo boring, perhaps an 807W/5933, or an earlier sweep tube for the final that works well at lower voltages would give it some snot.

I think it would be really cool to try a 4D32 for the series modulator, as it has great emission and would have minimal voltage drop on saturation.  What's more, they can easily handle the high voltage of a series modulator.  If you want great positive peaks, stack the 800 volt supply of the Heath transformer on top of (in series with) the 250/300 volt low  voltage output, and now you have tremendous positive peaks.  

The big issue is making a linear modulator for clean audio and minimal IMD using a single series tube.  Must stay in the linear area of the transfer characteristic.  In order to reach that goal with my bigger design, I am building just the power supply and modulator (breadboard style) and substituting a big fixed resistor for the final, with a value equal to the modulating impedance.  Then take a variable DC supply (maybe brute force with a variac) and vary the grid bias on the series modulator, through a resistor.  This is a cathode follower feeding this big dummy load resistor, so no grid current involved.   Now capacitively couple audio to the grid, and watch the signal on the dummy load, both for the DC simulated plate voltage potential as you vary the DC bias, and the linearity of the AC waveform as you increase or decrease the modulating signal. This will give you a good feeling for which tubes will play linearly with a series modulation circuit.

Once that is finished, you have the task of swinging the series modulator tube grid with bias and audio.  I plan to use a triode or tetrode, with the plate directly connected to the grid of the series tube, and a plate resistor connected directly to the HV B+.  This gives you the AC swing and the ability to set the DC operating point in a manner similar to the screen modulating circuits with which we are familiar.  This driver tube has to handle high voltage, but very little dissipation due to the low plate current.

If you can find a tube that qualifies for clean audio, good output voltage and current at the positive peaks, the rest is easy.  Very little iron, easy to tune, and no more heat than a screen modulated rig.  It seems practical for a PW rig, but only make sense for QRO in the dead of winter.

One final thought.  A sweep tube for a final on the low bands should not need neutralization, and with its high degree of emission needed for the TV pic to fill the screen, it should provide great output power and positive peaks without a great deal of modulated B+, probably much better than a 6146 or 807.  Do you have some left over from that 4x4 PDM rig that exited your shack?  You might even find a sweep tube works great for the series modulator as well, or maybe a pair in parallel?  GL and have fun with something COMPLETELY DIFFERENT!

Edit:  An afterthought-  If driving the series tube with bias and audio proves too complicated with it above the final, put the final directly below the power supply and modulate on the cathode side.   Return the final grid bias circuit to the cathode of the final, not the power supply ground, so that you have true series modulation and not cathode modulation - UGH!
This makes driving the grid of the series modulator simple and customary.  No need to float anything but the final tube and filament.  Capacitively couple the input and output RF, you just need caps that handle the high voltage, you have plenty.  Pi net output is standard, nothing out of the ordinary with the final on the top HV.  Grid circuit is similar too, with either grid-leak, or an isolated bias supply, which is not that much offset voltage for typical transformers to generate the fixed bias.  Have Fun!


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on September 03, 2020, 08:17:18 PM
Hi Rick,

You have me more interested in series modulation. I like the idea that there are very few guys doing it, so it will be a novel project and take some experimentation to get it running right.  Not counting a few tube PDM rigs, I don't know of any except for W2DTC with his 3CX-3000A7 rig.

You said:
"If you place the series tube above the final, the only thing you need to isolate is the tube socket, and use a filament transformer with very good secondary insulation with high breakdown voltage."

What is meant by placing the series modulator "above the final?" Do you mean in the same place as a conventional mod xfmr?  If so the audio drive circuit would need to be insulated with fiber optics or equivalent to keep the HV off it, right?  Maybe you meant something else.

I have some 6LF6 sweep tubes and some 4D32s that could be a good start in the series modulator.  Yes, an 807 would be plenty enough as a class C RF final.  I would like to put as much HV on the rig as possible to have that headroom.  I had about 2200VDC on my little 4D32 PDM (series modulated) rig, so I know the 4D32s will take the voltage abuse.   That will be a little much for an 807.

** Rick:  Could you make a hand-drawn schematic of your best ideas for a 30 watt series modulated tube rig?  I think between the two of us we could come up with a workable design that could later be tried by others once the bugs are worked out.  There seems to be a lot of interest in a series modulation based on the long threads in the past.   30 watts carrier out or so would be quite buildable by most guys here.


T




Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: KD6VXI on September 03, 2020, 08:41:59 PM
I believe he's talking about the difference in putting the modulator tube between the plate and B+ supply, vs the cathode and B-.

--Shane
KD6VXI


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: WBear2GCR on September 03, 2020, 08:44:08 PM
Sweep tubes, too big!

I vote for paralleled 12AU6 or 6SL6, if you want lo mu. Parallel a few...
Feel free to improvise on the tube types.

Complete heresy.

                      _-_-bear


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on September 03, 2020, 11:46:04 PM
I believe he's talking about the difference in putting the modulator tube between the plate and B+ supply, vs the cathode and B-.

--Shane
KD6VXI

I often thought of doing that with a big PDM rig.  There would need to be a fiber optic isolation to feed the audio into the grid/cathode of the modulator. And the B+ would connect to the CT of the modulator filament xfmr.  Or is there some other way?

Maybe in the end placement at the bottom between the cathode and B- is easiest.  But then the final is hot. Pick your poison.

T


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: w8khk on September 03, 2020, 11:58:23 PM
Hi Rick,

You have me more interested in series modulation. I like the idea that there are very few guys doing it, so it will be a novel project and take some experimentation to get it running right.  Not counting a few tube PDM rigs, I don't know of any except for W2DTC with his 3CX-3000A7 rig.

You said:
"If you place the series tube above the final, the only thing you need to isolate is the tube socket, and use a filament transformer with very good secondary insulation with high breakdown voltage."

What is meant by placing the series modulator "above the final?" Do you mean in the same place as a conventional mod xfmr?  If so the audio drive circuit would need to be insulated with fiber optics or equivalent to keep the HV off it, right?  Maybe you meant something else.

** Rick:  Could you make a hand-drawn schematic of your best ideas for a 30 watt series modulated tube rig?  I think between the two of us we could come up with a workable design that could later be tried by others once the bugs are worked out.  


Hi Tom,

Yes, the series tube is placed between the HV  positive terminal and the plate circuit of the final amplifier.  Whereas the Class-B modulation transformer adds to and subtracts from the Plate supply (adding power) the series tube just reduces the plate voltage via its resistance and the power is lost in plate dissipation.  It is the same situation whether you place the series dropping device in the positive supply, or in the negative supply between the cathode of the final and B- supply.

The big difference is how you provide grid control on the series tube.  Yes, it is simpler in the cathode side, and that might very well be the easier solution, but then the RF stage needs to be isolated, perhaps adding more complexity.

There is no need for opto-coupling to the grid of the series tube, even if it is placed on the positive side of the final amplifier.  Since no grid current is needed, it is simply necessary to provide a DC bias voltage on the grid to determine the operating point at carrier with no modulation, then add the AC component at the grid for whatever degree of modulation you determine appropriate.  The series tube then works as a cathode follower and provides the modulated plate voltage in step with the grid voltage variations.

Nothing exotic is needed in the driver stage(s).  Just another tube, resistors, and capacitors.  It works very much like the series regulated supply with a pass tube found in many of the older instruments, such as voltmeters generators, and oscilloscopes.  I will share some examples soon.

I will try to get time between other responsibilities in the next couple days to draw some diagrams and provide references of similar circuits,  with explanations.  If we take the design and validation a step at a time with some breadboard testing, I think it will be rather clear and easy for anyone to come up with a design specific to their desired voltage and power level with readily-available tubes.  Sweep tubes may be considered overkill for the 30 watt power range you are considering, but the available emission to perform well at lower plate voltages makes them shine as final amplifiers in this application.  

You did not mention whether you wish to go modern, or retro with regard to the tubes chosen.  I usually try to run what I have rather than go out and obtain something else, and you would probably prefer to do the same.
73..........


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on September 04, 2020, 10:39:20 AM
OK, Rick, sounds like a plan.

One thing I still don't understand...  If the series modulation tube is put in the B+ line, then its cathode, grid and plate structures are tied into the HV potential. How do you isolate the audio drive signal from ground? Do you float the audio driver itself or use large coupling caps to ground to avoid the HV DC or what?   It seems that somewhere along the line before the microphone that HV isolation is needed.  That's why I was considering the fiber optics.

I'm not sure about what tubes to use yet since there has been some discussion that some are better than others for linear curves, emission and best IMD.  I would think that the series tube should be at least three times bigger in dissipation than the RF final.  My 6AQ5 series modulated rig used just one tube in the modulator and it used to cook at 5 watts. I really needed two or a bigger tube in that circuit.

A single 6LF6 RF sweepie,   -     and single 4D32 series modulator still come to mind as a viable pair.   Lots of emission, RF capability and audio headroom.  Both tubes wud take huge HV like 1500V.  Even a single 807 in the final might be a better match for the 4D32 modulator dissipation-wise. I don't know yet.

T


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: w8khk on September 04, 2020, 02:11:42 PM
OK, Rick, sounds like a plan.

One thing I still don't understand...  If the series modulation tube is put in the B+ line, then its cathode, grid and plate structures are tied into the HV potential. How do you isolate the audio drive signal from ground? Do you float the audio driver itself or use large coupling caps to ground to avoid the HV DC or what?   It seems that somewhere along the line before the microphone that HV isolation is needed.  That's why I was considering the fiber optics.

Hi Tom,

I did some prep work, drew a simple modulator schematic, and researched some similar power supply circuits that do basically what we want to do.  

The driver will simply be a triode, tetrode, or pentode, with the plate resistor tied to the HV B+, and the plate tied to the grid of the series modulator tube, which works as a cathode follower.  The series cathode follower having a DC voltage gain of about .9, the driver plate must swing the AC voltage on the grid of the series modulator a bit more than the final amplifier sees in the modulation swing.  All we need here is a tube that can withstand a large plate voltage, but very little dissipation, such that it can develop the drive voltage across the plate resistor.

I will probably need to do several posts to get the sequence of steps to design your modulator down on the thread.  It would be so much easier with a whiteboard and a zoom meeting, but this will work.

The design needs to work backwards, qualifying and characterizing the series modulator tube, then the driver, so that we will know just how much bias and audio are required at the input to obtain your desired performance characteristics.




Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: w8khk on September 04, 2020, 02:19:30 PM

A single 6LF6 RF sweepie,   -     and single 4D32 series modulator still come to mind as a viable pair.   Lots of emission, RF capability and audio headroom.  Both tubes wud take huge HV like 1500V.  Even a single 807 in the final might be a better match for the 4D32 modulator dissipation-wise. I don't know yet.

We can use one or more parallel tubes in the series modulator to distribute the heat and dissipation.  We might prefer a sweep tube for the RF final, such that we can obtain our desired 25 or 30 watts with less un-modulated plate voltage on the final, thus allowing the series modulator to use the reserve voltage to generate higher positive peaks.  Or we could just be happy with plus and - 100 percent peaks, and run the plate supply at just over double the voltage to the final.  This operating point will be adjusted with a pot, and can be varied as desired.  You might use one or two 4D32s for the mod tube, with the screen tied to the plate with a resistor.  In my tests, I am starting with an RCA 810 or two for the modulator, and an RCA 8000 for the final. Possibly a 4-65 for the grid driver tube, lots of voltage, very little current.  My goal is to get the 3CX3000F3 working as the modulator for a pair of 304-TLs or something similar, with 2000 volts at 500 mA on the final, probably using a 4-250 as the driver tube, just to handle the voltage swing on the grid of the series modulator.  Plate supply will be 5000 volts at 1 AMP CCS from a Collins 5KW FM transmitter.


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: w8khk on September 04, 2020, 02:35:04 PM
I am sharing snips of portions of the power supply sections of Hewlett Packard 606 and 608 RF generators.   Our series modulator will work in a similar fashion to the series pass tube(s) in these regulators.  

The plate of the series regulator is tied directly to the output of the rectifier/filter.  The cathode of the series tube feeds the power supply loads, and in our case, the Final B+ is derived here.

There is a control tube that sets the grid voltage on the series tube.  It is simply a DC voltage amplifier, with the output taken from the plate resistor, directly coupled to the grid of the series tube.  The source voltage for this stage is the high voltage.   By varying the grid voltage on the control tube, we can take the series tube from cutoff to almost saturation, assuming we tie the cathode of the driver tube to a somewhat negative voltage source.

In the regulator circuit, the error voltage is taken from a voltage divider and pot to the control grid of the control tube.  As the output voltage rises, the control tube grid voltage rises, causing an increase in plate current, and a reduction of plate voltage.  This causes the voltage on the grid of the series tube to decrease, thus decreasing the output voltage and maintaining regulation.

Instead of this feedback loop, we can control the grid bias on the control tube with a pot, using some fixed voltages at each end, thus giving us a control to set DC plate voltage on the final, and easily adjust output power level.

Notice C44 in the 606 schematic, or C41 in the 608 schematic.  It is there to couple the output ripple directly to the grid of the control tube.  This ripple is coupled across the 1 meg DC bias coupling resistors, R78 or R69, respectively.  This allows the regulator to clean up the ripple in the output due to the phase inversion of the control tube.

We can insert our audio in this manner to override the DC set point, thus controlling the AC component of our series modulator.  The issue here is to obtain linearity in both stages as closely as possible.  The cathode follower should be relatively clean throughout its entire range up to but not including saturation.  We will have to play with different driver tubes to see which ones provide a linear transfer over the voltage range needed at the grid of the series modulator tube.

Next I will present the simplified modulator circuit, and explain the design process.  Most of it will entail pine-board bread-boarding, Frankenstein style!



Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: w8khk on September 04, 2020, 02:46:48 PM
I drew a simple two-stage series modulator, not to be constructed as drawn, but simply to describe the design process.

Notice the series tube will receive around 700 volts (or more) on the plate, and will be set to provide around 300 volts on the cathode, at 100 mA, such that a sample power input to the final will be around 30 watts, with a modulating impedance of 3000 ohms.  To start the design testing, we take a 3000 ohm resistor at around 50 watts to simulate the final power supply load.

We add a meter and a scope so that we can observe the output of the series tube.  A bench supply with a variac may be used to start with 300 to 400 volts on the modulator plate, increasing the voltage as we define the control voltage needed on  the grid.

A separate supply can be used to vary the output tube's grid voltage, such that we can look at the modulation peak and valley drive requirements with various plate voltages, DC operating points, and try different tubes or pairs of tubes.  We can either graph the AC requirements for modulation by stepping the input voltage (all this being done without the driver tube), or we can just determine the end points (max and min voltage out), close to saturation and cutoff, respectively.

Once this is completed, we can think about what driver tube can handle the voltage swing needed on the series tube grid, hopefully getting out of the 5 to 10 watt pissweakerland up to the desired 25 to 30 watts.  This should be very easy with a sweeper tube for the RF final and one or two 4D32 tubes (rated at 50 W dissipation each) for the series tube.  If that fails, you may always go to one of your spare 8877/3CX1500 tubes.    I know you have a nice stack of those!

If you use a tetrode or pentode for the series tube, just use a resistor from HV to the screen, similar to the way you would provide self-modulation of the screen in a plate modulated tetrode stage.  That is the way most of the series regulators work, and the value of this resistor can make a big difference in the voltage swing needed on the grid of the series modulator.


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: w8khk on September 04, 2020, 03:04:49 PM
Now that we know what DC operating point we need on the series tube to arrive at the desired static plate voltage, we can start designing the control tube as a DC amplifier, then add the AC input in the form of modulation.  
This stage looks very similar to the clamp tube circuit used on the AM transmitters with 807s or 6146s, and functions in much the same manner.  It requires very little plate current to swing the voltage on the plate resistor, but it must be good for a very large plate voltage potential.  Here maybe another sweep tube, or other high voltage triode or tetrode.  A tetrode or pentode or beam-power tube will require much less audio swing at the input to provide the desired modulation percentage.  Even though this may be a larger tube than a typical receiving tube. it will not run hot at the very low plate dissipation.

Case in point, back in 1965, I needed to build a power supply for my new Heath SB-101, and I was not able to buy the typical HP-13 supply from Heath.  I designed around a very large TV power transformer that provided almost 1000 volts in a bridge configuration, and 500 volts from half the bridge.  I designed two series regulators around that transformer, one for the 300 volts, using a pair of 6L6 pass tubes in parallel, controlled by a 6AU6 with an 0A2 reference tube.  I provided a series regulator for the 800  volts to the SB-101 using a pair of 6336 dual triodes as series tubes.  These are carbon plate dual triodes on steroids, with about double the dissipation and current rating of the 6AS7/6080 tube.  I was able to provide a solid, stiff source of 800 volts, adjustable, using a single 6146 as the control tube with a 100 Kohm plate resistor to the 1200 volts.  I tied another 100K resistor from the 1200 volt supply to the 6146 screen, and added a screen bypass resistor.  This supply was run for over 20 years with no tube or other part replacements.

For the control tube in the 30 watt rig, perhaps a sweep tube from a very small TV would work, as they can handle enormous amounts of plate voltage due to the way the TV flyback circuit operates.  Very little plate current, but lots of plate voltage will be found here.  A tetrode or pentode instead of a triode will result in significantly reduced input modulator voltage requirements, due to the higher gain of the screen-grid tube.  But more important than gain, is the linearity of this class-A stage, and reduced drive requirements (just voltage) should not be a priority over linearity over the entire modulation swing.

We can discuss this in more detail as needed.  As I finish up the final version of my MAX Audio Processor QC work, I will then have more time to spend on my series mod rig.  The final rev PC boards for the MAX arrived a couple days ago and I need to get to work on that almost full time for a while.  GL on the PW project, and stay tuned!

By the way, looking at some other threads, specifically one on Cathode Modulation, I discovered that Steve, WA1QIX, stated that he has successfully homebrewed several series modulated rigs, mostly before he focused on the PDM and Class-E designs.  He might be able to chime in and offer some great suggestions to short circuit some of the research and development time testing tubes and determining the best ones for modulation linearity.

Edit:  Have a look at reply 21 on this thread:
http://amfone.net/Amforum/index.php?topic=35443.0


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: WD5JKO on September 04, 2020, 06:10:01 PM


Here is another, series modulation with some power FET's, and DC feedback with adjustment pot to set the final RF stage B+.

http://amfone.net/Amforum/index.php?topic=27856.0

Jim
Wd5JKO


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: w8khk on September 04, 2020, 07:26:35 PM


Here is another, series modulation with some power FET's, and DC feedback with adjustment pot to set the final RF stage B+.

http://amfone.net/Amforum/index.php?topic=27856.0

Jim
Wd5JKO

Jim, thanks for sharing that info and thread!  I forgot that was available.  I may consider using some of his solid state design in a hybrid to drive my hollow-state series modulator.  It would be nice to just go with all vacuum tubes, but it might not be as practical as the hybrid approach.  The devices are still available.  I continue to be amazed at the higher and higher voltages the FET devices can withstand!


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on September 04, 2020, 08:23:41 PM
Wow - appreciate all the good info, Rick!

I'm very interested in your circuit below with the class C final at ground and the series modulator on top.  Maybe a 6LF6 sweep tube in triode configuration as the series modulator would work well. Let me digest this stuff some more and see what looks best.  I might even consider a somewhat bigger rig, dunno yet.  Triodes  in the modulator (as you show) wud be simplest. A tetrode RF final wud modulate best.

http://amfone.net/Amforum/index.php?action=dlattach;topic=46165.0;attach=65922;image

Later  -

T


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: w8khk on September 05, 2020, 02:25:40 AM
Maybe a 6LF6 sweep tube in triode configuration as the series modulator would work well. Let me digest this stuff some more and see what looks best.  I might even consider a somewhat bigger rig, dunno yet.  Triodes  in the modulator (as you show) wud be simplest. A tetrode RF final wud modulate best.

I believe a single or pair of 4D32s in the series modulator would be as simple as a triode, just one more resistor for the screen.  With a pair you could probably run more than 50 watts input to the final.  Kinda hard to find another tube with the emission, plate dissipation, and voltage tolerance of this Raytheon Wonder!  It's no wonder you need almost 8 amps filament current for a pair.

Agreed on sweep tubes for the RF, as they excel in producing lots of emission and power at lower voltages, improving your ability to get larger positive peaks.

As for the driver tube, tetrode or pentode would give more gain and require less audio input voltage, but possibly at the expense of linearity.  It is EASY to get modulation VOLTAGE, as there is really no load here, no real power required, just higher level.

If you proceed with testing various series modulator tubes for static output into a simulated 3000 ohm load, I would be very interested in your findings as to the source plate voltage you choose to test with, and the grid vs output voltages from standing carrier to peak and valley.  We do not need all the steps taken, just the three, and the rest of the linearity evaluation can come with the audio input scope output evaluation.  

I believe the series modulator cathode follower will be very easy to get decent linearity, but the driver tube may require some juggling and some study of the tube handbook recommended values for audio amplifier designs.  I doubt they will go anywhere into the voltage ranges we are interested in, but we should be able to extrapolate beyond their values based upon mu or transconductance.  I have found simulation to be very inaccurate at these extreme voltages compared to typical audio amplifiers, but various wave-forms will tell the story very quickly as we vary the bias (operating point) and the audio level input.

Charting some of this data in tabular or spreadsheet format may prove valuable for future series-modulation addicts.

Based upon Jim's (JKO) recent post on the thread, I would certainly consider MOSFETs as an alternative to the driver tube, but I would probably retain the series tube to allow extension to voltages well above PissWeakLand.

To reiterate, my preference to series over screen modulation is the ability to run a standard modulated class C final with ease of tuning and clean modulation, moving the inefficient dissipation issues to the series tube which entails no critical adjustment to ensure a clean signal.  I gladly trade the power wasted in dissipation for the lack of response-limiting and FRAGILE modulation iron, and it is a welcome breath of fresh HOT air in January, especially in your neck of the woods!  My Motto:  Series modulation is 100 percent efficient in heating the winter shack, but you gotta be an Old Buzzard for it to be effective!  Class E and PDM rule in the summertime.

GL and 73!


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on September 06, 2020, 12:00:36 AM
Hi Rick,

I have decided on a tube lineup and some details for the new series modulated rig.  I wanted to keep you up to date so we can focus.  I was talking with Frank/GFZ tonight and came up with the following ideas:

A single 4D32 RF final in class C - series modulated by FOUR 6LF6 sweep tubes in parallel is the current idea. The modulator would be at ground reference, below the RF final, in the cathode lead, just like conventional tube PDM rigs.   I would tap an adjustable 1-2 KV HV from my Baby Blue 3-500Z amplifier to keep the new rig small. I could use a plate to screen dropping resistor on the final to eliminate the screen supply.

I won't get into the calculations now, but the rig should put out about 35 watts carrier with big, clean, positive peaks.  Frank suggested a 15 ohm, 5 watt cathode resistor for each 6LF6 tube to balance the load and provide a small amount of NFB.

He suggested a resistor across the modulator plate to cathode to act as an NPL.  The 6LF6 control grids will be grounded and the screens driven by a single-ended 11N90 class A MOSFET driver.  The 6LF6 sweep tubes are robust 40 watt tubes. The 4D32 will be loafing as a 50 watt RF tube.  The modulators should be OK heat-wise with a slight breeze.

I will need a HV insulated fil xfmr for the 4D32 floating cathode circuit.

The overall idea is to replace my FT-1000D with a homebrew AM driver. This AM exciter can drive my 4-1000A linear amplifier no problem or act as a stand alone PW 35 watt rig.  Once optimized, the audio should be superb and the RF final will tune easily like any other class C plate modulated rig.

The math shows it should work OK. I will post that later once I am sure this is the right path.

On the subject of an NPL.... my series modulated 6AQ5 rig didn't have a formal NPL but it limited well at the negative peaks like it had one. I could pour on the audio to 200% easily. I think it was the string of diodes I used to set the carrier in the cathode of the modulator.  That may have acted as an unmodulated signal just like the feedthru power of a GG linear amp, but I am not sure.  

http://www.amwindow.org/tech/htm/series.htm


More later.


** Here's the 4D32 and 6LF6 tube data sheets:

https://frank.pocnet.net/sheets/201/4/4D32.pdf
https://frank.pocnet.net/sheets/084/6/6LF6.pdf



T


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: w8khk on September 06, 2020, 04:26:28 AM
Tom, that topology should prove very successful.  The 4D32 will have lots of headroom, and we know the screen will self-modulate well with a series resistor.  If you were running only 800 to 900 volts from your supply, then a sweep-tube final would have made it easier to get nice big positive peaks.  But with the potentials derived from the new baby, the need for the sweep-tube final evaporates.  I do like the 4D32 for RF, and it will take everything you throw at it if you need to clear the aether!

You may use grid-leak bias to keep it simple, or also include a fixed bias supply that floats with the 4D32 cathode.  For the heater, 3.75 amps at 6.3 volts isolated is easy.

I like the three 6LF6s with the combined dissipation of 120 watts, and they are small too.  

With transformer-coupled plate modulation, the modulator output adds to and subtracts from the plate voltage, so negative over-modulation can readily occur.  Because the series modulator is in series with the supply and does not add any voltage to the load, the lowest it can go is zero, at modulator cutoff.  The resistor across the modulator should be a very effective way to guarantee that there is a slight amount of supply voltage to the final, even at modulator cutoff, ensuring no zero-crossing.  Simple, elegant solution!

I have never played with the 6LF6, but you and Frank have lots of experience there.  The balancing resistors and screen drive should provide a robust solution, and the variable supply will enable lots of positive peaks, while running the 4D32 in loafing mode, compared to a 32V.

As it comes together, please share the design of the FET driver, as well as the bias set point circuit for carrier level control, and the audio path and input requirements.  This one is sure to be imitated by many.

Floating the 4D32 at the voltages you propose is straightforward, and the input/output RF networks require no special attention.   On the other hand, the rig I propose with the 3CX3000F3 and the P-P 304-TLs is a bit more challenging when floating the RF, especially since the RF deck is already restored from the one my dad built in 1949.  So I still plan to pursue the modulator-on-top architecture, running the 3CX as a cathode follower, and driving it with a 4-250 or 4-400 to handle the high plate voltage.

Just curious what you propose for frequency control?  The ricebox, or a DDS, ???  Self-contained would be sweet, but I think more important is ease of zero-beating with other folks, so other alternatives may be superior.

Thanks for sharing your plans, it should sound great with the iron stripped out of the signal path!


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on September 06, 2020, 11:11:47 AM
OK Rick -

I changed the 6LF6 series modulator count to FOUR tubes.  Frank suggested that the  operation point may drift around as the tubes heat up. It's probably best to have extra headroom to play with since the 4D32 final is just loafing.  Maybe it will be more of a 40 watt stand-alone PW rig, but I can drop it way down to drive my three bigger linears (or 20 watt PW use) and still have the potential for big audio peaks at lower voltages below 1KV with the high-emission 6LF6s.    My Baby Blue  HV supply transformer taps and the Variac will give a lot of options. Four 6LF6 40 watt class A sweep tubes (160w diss) feeding one 50 watt class C 4D32  RF tube with a wide range of HV variation shud give the platform good utility.

I plan to use the FT-1000D as a VFO as I do with all the rigs in the shack. The 4D32 won't need more than a few watts to drive it.

Yes, grid leak is FB for the 4D32. I also have an insulated fixed grid bias xfmr that I wound -  left over from the old 4D32 PDM rig.

This should be an interesting project.  Thanks for your input to get the boat headed in the right direction. It will come together slowly over time... I will put together a hand drawn schematic as a first step - so we can critique it here soon.

T




Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: w8khk on September 06, 2020, 11:41:41 AM
Tom, I am liking it even more as the design grows.  I would very much like to see what you and Frank come up with for the audio input and carrier control (DC set point) circuits.

Another positive in adding a fourth series modulator - Not just the power level you may run, but also the cleanliness.  Remember you are running class A on the series tube, and it is NOT a cathode follower when placed below the final amplifier.  It will not be perfectly linear throughout it's entire grid-plate swing.  So having extra current capability and dissipation, as well as a lower impedance source by virtue of more parallel paths, you will be able to run at a lower level of audio swing (percentage wise) and stay within the linear portion of the curve when modulating.

I wonder if you plan to do any preliminary modulator testing before building it all up with the RF section?  Easy to tie one, two, three, or even four tubes together, without any chassis, and apply power, audio, bias, and a load.  Figure what your target modulating impedance (plate volts / plate current) will be for your final, and substitute this resistor between plate supply and modulator plates.  Now add your cathode resistors to ground, connect a variable DC supply to the screen grid through a resistor to provide bias (a variac supply is fine here) and inject some audio with a coupling cap from an audio generator and amplifier, if necessary.

Scope the output at the plate through a blocking cap and resistor, and look also at the DC voltage at the plate.  Play with the inputs to see what voltage comes out (peak to peak) and how it varies in performance at different dc set points for input power selection.

This is a very simple procedure that will give you real numbers for your actual tubes, and help to plan actual power levels and audio cleanliness before you even create RF.

I am doing the same thing with my 3CX3000, just testing the series modulator tube, no driver circuits as yet.  My load is a string of either 16 or 20 each 60 watt incandescent lightbulbs, for 1920 volts at 960 watts input, up to 2400 volts at 1200 watts input.  With a fixed DC set point, the bulb resistance is rather stable, and it does not change much with modulation, but with the voltage measurement and known load dissipation the watts are obvious, and the scope tells how clean the output wave form is when passing through the thermionic transconductance.   This is all done with a variac on the 5000 volt 1 amp CCS supply.  The blower on the bench for the 3CX tube is quite noisy!

You might find a string of smaller light bulbs makes a good load for your modulator test.  I think it is worth the time so that modulator and RF issues do not have to be addressed simultaneously in the end.  I think the linearity of modulation is a much higher priority than just the estimation of the raw power output. 

Another consideration is that if you do run into linearity issues when it is all together, you know where you stand with the series tube performance, and you can then address fine-tuning the audio driver circuits, plus, you will already know what bias levels you need to provide for the desired input power ranges.  You might be able simulate all this or calculate it in advance, but with four tubes from the dual quads, and the higher voltages, seeing the triangle and sine waveform and measuring the voltages is probably somewhat more practical an approach.


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: WA1LGQ on September 06, 2020, 11:43:44 AM
I just dug out my Cheyenne transmitter the other day, basically a DX-60 with a built in VFO. I need to come up with a power supply for it first. Bill, we were talking about this rig a couple weeks ago on 50.4, you had inspired words to say about the screen modulation which I have not done in decades. Also, thinking of modulating my "BC-3.75" mini hybrid mil rig, and thought of suppresser grid modulating a 6AG7 also. How much power did you get out of yours Mike?
Larry


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on September 06, 2020, 01:08:14 PM
Tom, I am liking it even more as the design grows.  I would very much like to see what you and Frank come up with for the audio input and carrier control (DC set point) circuits.

Another positive in adding a fourth series modulator - Not just the power level you may run, but also the cleanliness.  Remember you are running class A on the series tube, and it is NOT a cathode follower when placed below the final amplifier.  It will not be perfectly linear throughout it's entire grid-plate swing.  So having extra current capability and dissipation, as well as a lower impedance source by virtue of more parallel paths, you will be able to run at a lower level of audio swing (percentage wise) and stay within the linear portion of the curve when modulating.


Rick,

I'm about to take a slight detour...

Youse gots me concerned about the overall linearity of these four 6LF6 tubes.  Yes, staying on the straight part of the curve is important. I'm thinking it will be even harder to do with four tubes. They are not matched or by any means perfect. They are used and abused from the dual quads PDM days. The whole purpose for this project is perfect audio with heat as the tradeoff - so linearity issues are not acceptable.

It may sound crazy, but a single 3-500Z triode, (a 500w diss tube) in place of the four 6LF6s sounds like a better/cleaner  solution to me. At 1500V it is -46dB 3rd order IMD, the best in the world,  and the curves look straight as an arrow.  The power output of a single class C 4D32 now comes up to an easy 100-125 AM carrier watts out.  The overall efficiency is not much different than running the 3-500Z as a linear. I get out about 100-150 watts with my Baby Blue single 3-500Z or the Summer Breeze 4D32.  

We may need to add to the 3-500Z a resting bias that goes positive when at lower voltages, I dunno..


Check out the beautiful specs and curves:
http://w7brs.com/3500z/3-500z.html

This reminds me of the scene in Animal House where the unwanted pledges (like Flounder) always end up in the same room with the other undesirables.  No matter how hard I try to build a PW rig, I always end building a 100 watter.  "Sit down with the other 100 watt pledges and enjoy yourself, Tom!"

https://www.youtube.com/watch?v=LuFCaIAnETk

From a pure performance point of view, a single 3-500Z class A series driving a single 4D32 class C with 1-2KV would be hard to beat.   I have an extra 3-500Z and chimney/socket in the stash.  I don't think there would be a linearity problem running the rig below 100 watts.   I don't plan to do any pine board pre-testing, rather build it and improve from there.  Choosing the best tube lineup from the start is important.  I can envision  both the 3-500Z and 4D32 loafing along at any power level between 30 to 125 watts with a turn of the Variac - and the ability to do 200%+ audio peaks with the NPL if desired. That's the kind of dream worth building.

T


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: w8khk on September 06, 2020, 01:35:27 PM
I think we need to take a step back and I should explain how simple the issue is.  I do not think you should abandon the sweep tubes for a 3-500Z.  That is like bringing in a bulldozer to kill an ant.  And you want it quiet, small, no blower noise, etc.

The issue is not the non-linearity of one tube vs another, or old beat tubes vs new.  In parallel, the curves average anyway.



What I am speaking of is using the most linear part of the transfer curve, no matter what the type.  Toward saturation, the curve levels off.  It is a matter of setting the bias and operating point where you will stay on the straight line without your peaks being compromised.  I dare say the 3-500Z is way overkill, and if you were going to do that, just modulate a pair of 813s!

The sweep tubes are really a perfect fit for the application,  as they withstand very high plate voltage, they have outstanding emission for peak plate current, and they will provide higher plate current at lower voltages than most tubes.  I think you have an excellent plan.

I was only trying to provide another method of "getting down the numbers" so that when you design the operating points and the driver circuitry, everything is in the best part of Class A.  I have no doubt the sweep tubes will do a clean job, my only point is whether you do real-time analysis before you build the unit, or fine-tune the parameters after it is assembled.  

I should have realized from history that you prefer to fine-tune after the fact, and run everything in the sweet spot.  I more often breadboard and come closer to the final design before I punch the chassis.  Different strokes.  I would have faith in Frank's suggestions, leverage his ideas and design of the input circuitry with the FET driver, and have fun.  Save the 3-500Z for another project or as  spare.   I hope I did not derail your enthusiasm.  Press on, have fun!


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on September 06, 2020, 02:54:18 PM
OK on all, Rick -

So, it appears the 6LF6s will be fine as long as I find the best part of the straight operating curve thru experimentation.  I can do that no problem.

Two points about using the 3-500Z -  one is that the filament power is reasonably close for both.  72 watts for the 3-500Z  vs: 50.5 watts for the four 6LF6s.  That is the only advantage the 6LF6s have that I can see. I could put a small breeze thru the 3-500Z to keep it cool enuff without much noise as I do running my 3-500Z linear now.

Also, the 3-500Z modulator gives me the option of running 125+ watts carrier out with the single 4D32 whereas with the 6FL6s I am pretty much limited to 45 watts out.   I am using  30% X the mod tube dissipation as the calculation.  IE, 3-500Z, 500 watts X .3 = 150 watts out.       6LF6,  160 watts X .3 = 48 watts.  The 4D32's full potential is being utilized when the 3-500Z is used.

I am assuming that the other performance areas are about equal. I do not have to contend with a screen with the 3-500Z.   So overall, using the 3-500Z is going to waste an additional 22 watts of filament power but give the option of running 100-150 watts AM carrier instead of being limited to 48 watts max. I assume I can reduce the HV and get a range of 30-125 watts carrier, but we will have to see. The tube has a proven linearity design, whereas the 6LF6s have sketchy data, though they are probably OK for the job.

Thoughts?

T


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: KQ6F on September 06, 2020, 03:15:27 PM
Tom -

I don't have a dog in this fight, but as a casual observer of this thread it sounds to me like a good way to go would be to pine-board both modulators along the lines of what Rick has suggested and see which works best...

Just my 2 cents.

Rod


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: AG5UM on September 06, 2020, 04:44:00 PM
Tom,
Been following this discussion, trying to learn as much as possible, very interesting ideas here.
I thought you were designing a 30-40 watt transmitter to drive a linear, that seemed like a cool idea,
For many of us, driving a linear with 100-150 watts means using an attenuator, dummy load,etc. and wasting a lot of power.
I realize with your skill/knowledge level you seem to be able to make any transmitter operate properly at any power level,
...Still it seems it would be useful to have a 30-40 watt transmitter just to drive a linear,etc.
Anyway, whatever you come up with I know it will be interesting, keep-up the good work..
I'm also enjoying learning more about Sweep tubes, interesting...
Donnie  AG5UM


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: w8khk on September 06, 2020, 04:46:54 PM
You raise many different issues that would need to be evaluated with some calculations or simulation before any obvious answers can be had regarding the switch to the 3-500Z.  It appears your goal of having a pisweaker to run either barefoot or to drive a linear has migrated to having a general purpose AM transmitter with variable power output to the limit of the 4D32.  While this is not a problem, it will take some time to digest and provide meaningful suggestions.

First, the filament power and fan cooling are so small so as not to be a factor.  If size and weight are not a concern, then you pretty much have a clean slate.  But my concern, with the 3-500Z, is whether you will have a "friendly" to operate low power rig with such a series modulator.  It really has more to do with the voltage and power range for which the tube is designed, 

If we go back and look at the series regulated power supply designs, we can compare the basic trade-offs between tubes when looking at series regulators like the 6AS7-6080 vs the 6L6 and similar higher voltage tubes.  The 6AS7 has much lower plate resistance and lots of dissipation, so if a large current is desired from the supply, it can be produced with a relatively low rectifier output, because the series tube drops less voltage for a given output current than a tube with less emission and higher plate resistance.  So to produce a series regulator for a higher current with the 6L6, higher rectifier voltage output is needed, and the dissipation in the series tube goes up with the fixed output current and larger voltage drop across the series tube.

A similar situation occurs if you take an RF final amplifier and switch from 250-THs to 450-THs.  For similar output power, it is necessary to run somewhat higher plate voltage due to the wider electrode spacing. 

I have not made any reference to the tube curves for either the sweep tubes or the big Eimac triode, but I would expect that to achieve the same output current to the 4D32 final with the 3-500Z, you would need to run significantly more HV from the power supply than would be required with the sweep tubes.  Also consider the fact that you have several low-resistance sweep tubes in parallel running at a lower voltage, compared with a single 3-500Z with wide electrode spacing, the higher plate resistance equates to larger voltage drop across the series tube, greatly increasing the overall dissipation for the same RF power output. 

My first inclination is that the 3-500Z is not really a match for this application unless you are going to be running it to obtain maximum power from the 4D32, and you do not care how much voltage or heat you throw from the series tube.  For this application, the linearity of the tube itself is secondary to running the chosen tube at the DC operating point to realize linear audio amplification.  I did not intend to imply that the sweep tubes would not provide satisfactory linearity for audio amplification.

Likely I should not have raised much concern about the linearity of the series modulator tube with regard to the audio signal.  If you look at what happens to the signal between the microphone and the modulator input, you have a great deal of processing or tailoring of your signal.  EQ, compression, filtering, etc.  A slight amount of variation in the waveform in the stage that feeds the series tube will most likely be totally unnoticeable, assuming you are not running it into cutoff or hard saturation with modulation.  And a slight variation in the audio waveform will cause no more IMD in your signal than will your graphic equalizer that tailors you voice to your preferred shape.  Bottom line, with the ability to set the DC output level and power, as well as the modulation level, independently, the resulting sound should surely rival any plate modulated rig, and will probably be more linear than a screen modulated rig even with your used sweep tubes.  I really think there is nothing to worry about with the original plan you and Frank concocted. 

If I was in doubt, I would at least connect one tube, one series resistor, a meter, scope, and a couple variac power supplies and SEE the output waveform, as well as the input and output voltages need at your desired power level (plate current) to know where you stand before building.  Then you could make an educated guess as to how it would behave with multiple tubes in parallel through extrapolation.   I did all this with 6AS7s, 6L6s, and a couple other tubes with clip leads and a couple hours work before building an earlier series mod rig years ago.  I strapped on a few harbor freight DVMs, and did it all from a distance on the workbench, and tabulated the numbers.  I no longer have the data, but it was a very interesting exercise with simple receiving tube types.   I have all the tubes except the sweep tubes, I am getting interested enough to play that game in the next few days if it would help, but unfortunately no 'LF6 tubes live here.  With Frank's experience, I would expect that what he is proposing will be rather easy to find the sweet spot after you assemble it.  Can't wait to see that FET circuit design!

I will continue following this with great interest, but over the next few days I will be spending a great deal of time mentoring students with their "FIRST Robotics" program in south Florida, mostly via zoom meetings.  I also need to spend considerable time on the documentation and final QC for the audio processor project, as we have several folks patiently awaiting results!  The boards desire more solder smoke!


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: KD6VXI on September 06, 2020, 05:52:24 PM
What about a 4-125 modulating a 4-65 :)


Or a 4-250 modding a 4-125?

Not a huge fan of paralleling tubes.  If you need more than one tube, you need a different tube.

--Shane
KD6VXI


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on September 06, 2020, 06:05:31 PM
Thanks much for the comments and suggestions, guys!

This is exactly how I like to design stuff. Come up with a possible design on paper and then try to improve on it. Keep trying different ideas until there are enuff reasons to stay with the latest one.  No harm trying stuff on paper.

From what I am seeing, there are several reasons to stick with the quad 6LF6s series modulating a 4D32. The sweep tubes are simply better suited for the job.  My first reason for this rig is as a driver for the various linears. It will require 10-30 watts carrier. The sweep tubes are best suited for this. Second, I want a pissweaker rig. I have enuff 100 watt class rigs, but no PW rigs. The sweep tube modulator is best for this purpose. The 3-500Z modulator would be more suited for higher voltage as Rick says. I think keeping it optimized in the 10-40 watt range is the best choice here.  We can design up a matching MOSFET driver suited to this lower range.

And finally, I get the impression there are a lot of guys who might like to build this rig once proven, working FB and there is a schematic and operating procedures for it. Whether as a stand-alone 35-40 watt AM rig or linear driver at 10-20 watts carrier, this will make a great RF source with transparent audio.  Making the modulator a 3-500Z would knock some guys out of the game due to cost and generally larger parts needed..  It would also ruin the pissweaker mystique by making it 125 watt capable.

So, I will draw up a basic schematic for quad 6LF6 series modulators and a single 4D32 final and then get back with Frank and see what he can come up with for a simple MOSFET audio driver.  It looks like we will be grounding the grids of the 6LF6s and directly driving the screens with the MOSFET driver. No audio transformers anywhere. This will make it more stable and will simplify things further.  We might look into audio NFB (neg feedback) later on.

Thanks for the detailed info Rick and comments from the other guys here.

Look for more info on this project soon.

T


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: KD1SH on September 06, 2020, 06:25:55 PM
  Like mopeds - too fast for the side of the road but not fast enough for traffic - 100 watt rigs are awkward: too much to drive an amp but not enough to strap on their own.
 


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: KD1SH on September 06, 2020, 06:36:46 PM
Hi, Larry,
   I remember you mentioning your Cheyenne the other day on 6m AM. I've heard that while they're very similar to the DX-60, they're not exactly the same. To me, it's a much nicer looking rig than the DX-60.
   Hope to see you at Vintage Radio next Saturday!

I just dug out my Cheyenne transmitter the other day, basically a DX-60 with a built in VFO. I need to come up with a power supply for it first. Bill, we were talking about this rig a couple weeks ago on 50.4, you had inspired words to say about the screen modulation which I have not done in decades. Also, thinking of modulating my "BC-3.75" mini hybrid mil rig, and thought of suppresser grid modulating a 6AG7 also. How much power did you get out of yours Mike?
Larry


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: w8khk on September 06, 2020, 10:17:41 PM
What about a 4-125 modulating a 4-65 :)


Or a 4-250 modding a 4-125?

Not a huge fan of paralleling tubes.  If you need more than one tube, you need a different tube.

--Shane
KD6VXI

Shane, thanks for your suggestions.  I agree it is preferable to use one tube instead of several in parallel for the RF output, for many reasons - stability, neutralization, output impedance and capacity especially on the higher HF bands......   But here we are paralleling the series modulator, which in this  configuration actually offers more benefits  than running a single tube.  Providing a lower impedance source via parallel tubes provides better capability for providing the necessary plate current on positive peaks, and reduces the overall dissipation by allowing less voltage drop in the modulator for the same output voltage and current, thus making the whole affair more efficient.  They are easy to balance through cathode degeneration, as well as easier to drive as lower audio level is required.

The Eimac series of tubes you suggest are fine players at both RF and as modulators, but they sing much better at significantly higher voltages than the sweep tubes and the Raytheon 4D32, and when putting them all together in series, it would require a much higher overall power supply voltage to produce similar power levels with the 4-65 and the bigger siblings.  This would make components more expensive, require more complex isolation, and probably a larger package for the same overall performance in the series-modulated architecture under consideration here.  And  it appears we are still focusing on a low-end power level for either stand-alone QRP AM, or conveniently driving a linear to full scrote while getting the lead (-er Iron) out of the signal path.  What's not to like?


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: w8khk on September 06, 2020, 10:35:06 PM
This is exactly how I like to design stuff. Come up with a possible design on paper and then try to improve on it. Keep trying different ideas until there are enuff reasons to stay with the latest one.  No harm trying stuff on paper.

From what I am seeing, there are several reasons to stick with the quad 6LF6s series modulating a 4D32. The sweep tubes are simply better suited for the job.  My first reason for this rig is as a driver for the various linears. It will require 10-30 watts carrier. The sweep tubes are best suited for this. Second, I want a pissweaker rig. I have enuff 100 watt class rigs, but no PW rigs. The sweep tube modulator is best for this purpose. The 3-500Z modulator would be more suited for higher voltage as Rick says. I think keeping it optimized in the 10-40 watt range is the best choice here.  We can design up a matching MOSFET driver suited to this lower range.

And finally, I get the impression there are a lot of guys who might like to build this rig once proven, working FB and there is a schematic and operating procedures for it. Whether as a stand-alone 35-40 watt AM rig or linear driver at 10-20 watts carrier, this will make a great RF source with transparent audio.  Making the modulator a 3-500Z would knock some guys out of the game due to cost and generally larger parts needed..  It would also ruin the pissweaker mystique by making it 125 watt capable.

So, I will draw up a basic schematic for quad 6LF6 series modulators and a single 4D32 final and then get back with Frank and see what he can come up with for a simple MOSFET audio driver.  It looks like we will be grounding the grids of the 6LF6s and directly driving the screens with the MOSFET driver. No audio transformers anywhere. This will make it more stable and will simplify things further.  We might look into audio NFB (neg feedback) later on.

Tom, I agree on all counts!  I think together we have all shaken this in every conceivable direction and narrowed it down to a practical and fitting solution that addresses your original goals.

It will be above the pissweaker class enough that it will be usable on a regular basis, with ease of enabling the afterburner without switching to a different rig when a bug needs a good squashing.  It is practical in size, weight, and friendly regarding parts procurement. The isolation issues are trivial with the voltages present.  With careful layout and  ventilated enclosure, I would expect convection cooling would obviate any requirement for a noisy fan.  Four sweepers should run much cooler overall than just one, or two in parallel.

Even though you will use the hybrid approach to provide power and drive (Baby Blue and the FT), there is no reason why others could not clone the core design in a stand-alone package.  A DDS with an intermediate amplifier could drive a single 4D32 in grid drive configuration with ease, and and the power supply requirements are low enough in current demands that even a modest transformer with a voltage doubler could provide all the energy needed while coasting.  The other power supply components are small and inexpensive, so this design can be taken in many directions by others in the future!  What a contrast to a single 6AQ5 modulated oscillator!

Looking forward to seeing this come together with more of Franks's design as all your ideas unfold!


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: km6sn on September 07, 2020, 05:57:57 AM
OK, Ok True Heresy coming up here. Fasten your seatbelts.

If you goal is to use hollow-state and see fire in the hole, that is one thing.

However, if you goal is to make an economical clean pissweak rig to use barefoot or to drive a linear, here is another approach.

http://amfone.net/Amforum/index.php?topic=46102.new;topicseen#new

It turns out two 74ac08 gates in parallel are an ideal drive for 100 ohm twisted pair to drive a NCP81074
fet driver on 80,40, and 20. For the RF deck, a single Transphorm  TPSH34208 (TPSH34206 would be even better) in class D is all that is needed. Power is scalable by changing the power supply voltage for the modulator and Class D RF deck.

In the project shown, everything is on .1 inch boards with ground plane, or on bare copper dead bug  (live bug, in the case of the NCP81074).

Driving a 3-500 one-holer, I get broadcast-quality audio reports.

Steve WA1QIX suggested changing to an analog modulator, and I think I will do that. Thanks for the idea Steve.

The whole shebang will use less power than the 3-500 modulator filament! Less shack heat and audible noise.

OK, thanks for bearing up under this Truly Heretical post!

Regards,

Rod



Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: KD6VXI on September 07, 2020, 10:07:42 AM
Rick,

All good points.  I was making that recommendation based upon the voltage levels with Baby Blue, which iirc was 2kv or so max.  The 4 series tubes would have made a killer match up if he would have wanted to make it a 'companion'.

I've run 6LF6s at 1.2kv before, they really get up and go!  Great tube for sure.  The 6LF6, 8950 and 8908 where my all time favorite "back then".

Never really messed with a 4D32 but from everything I've read it's another killer.  The gubmint must have loved it for a reason, so....

One thing is for sure:  No matter what lineup Tom chooses, it's gonna be a nice rig!

--Shane
KD6VXI
--Shane
KD6VXI


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: w8khk on September 07, 2020, 12:37:44 PM
Tom, we have both seen these links before, we just did not remember....

If these do not give you the ultimate confidence in going with the quad of 6LF6 sweepers, nothing will.  Just strap in your Raytheon 4D32, then STRAP!

73, Rick

The thread:

http://amfone.net/Amforum/index.php?topic=4821.0

Article:

http://www.amwindow.org/tech/htm/wb9eckseriesmod.htm

Hi-Res Schematic:

http://www.amwindow.org/tech/pdf/eckfinal2.pdf


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: w8khk on September 07, 2020, 12:55:23 PM
Rick,

All good points.  I was making that recommendation based upon the voltage levels with Baby Blue, which iirc was 2kv or so max.  The 4 series tubes would have made a killer match up if he would have wanted to make it a 'companion'.

I've run 6LF6s at 1.2kv before, they really get up and go!  Great tube for sure.  The 6LF6, 8950 and 8908 where my all time favorite "back then".

Never really messed with a 4D32 but from everything I've read it's another killer.  The gubmint must have loved it for a reason, so....

One thing is for sure:  No matter what lineup Tom chooses, it's gonna be a nice rig!

--Shane
KD6VXI

Shane, I agree those Eimac sibling tubes would sing for their supper with the Baby Blue 2 KV supply all out.  If Tom was just going for a full-strap rig, those would be good choices.  We kinda got off-track and escalated the ideas well beyond his original intent, and we are back in focus now.  Thanks again for your input.

Going the other way, with much lower voltages, one thing I would love to try would be a handful of 6336 dual triodes for series modulating another one or two.  The 6336 is like a pair of 5AS7/6080 triodes in the same envelope, with double the heater current and huge graphite plates.  Perhaps that would make a nice rig with a low impedance plate tank wound on toroids.   So many project ideas, so little time.


Title: Re: SCHEMATIC and PICS - Looking for ideas for a ~25 watt Pissweaker rig
Post by: K1JJ on September 07, 2020, 02:23:57 PM
I drew up a preliminary schematic for the four 6LF6s series modulating the 4D32.  Take a close look and see if you can add or change any parts values or find mistakes.  Especially check the floating HV area of the 4D32 and the returns to the cathode rather than ground.

I am not sure if the GFZ driver carrier adjustment will substitute for the cathode diodes or whether they will help with NPL action too.  The modulator 100K from plate to cathode will be dissipating some power... maybe 250K is better.  We will want to look into that closer.  Summer Breeze runs just one 4D32 and can do an easy 100 watts out at 600V. So I am thinking that a total voltage of 1500V will be more than enuff to dial in 40 watts of carrier out and have reserve for huge positive peaks if desired. I have a Variac on the HV, so no problem testing for the sweet spots.

It's really a simple circuit and has only a handful of voltages needed:  Two filaments, one HV, one fixed grid and the MOSFET driver voltage, whatever that works out to be. Amazing, really.

The 4D32 grid and screen meters will have to be floated in Plexiglas, though the cathode meter is at ground potential.  I would add a HV meter too making it a total of four meters.  The grid and screen current will have to be mentally subtracted to get a true plate current reading. The screens of the 6LF6s could also use a meter, so let's make that five meters.  The 6LF6 screen current data sheet shows 70 mA each. That is a lot if we multiply times four tubes. But I remember my quad 6LF6 PDM modulator hauling down 200 mA screen current to make big audio peaks. It didn't hurt anything. The MOSFET audio driver will be feeding the screens, so no big deal to haul down current.

The RF choke between the modulator and 4D32 cathode should be between 0.5 to 1 MH or so. Testing will show what is invisible to audio and still work right.

I added a 5KV safety cap at the RF input to back up the other coupling cap in case it fails.

I am lining up some parts as you can see.  It will definitely be four 6LF6s modulating a single 4D32. This has the potential to be my most hi-fi, cleanest TUBE AM rig, ever. With the coming simplified GFZ MOSFET audio driver we have all bases covered. I'm hoping others will build it up in the future too.

T

***  LOOK   FOR A NEWER UPDATED SCHEMATIC -   Some important changes on 9-8-2020, 9 posts down.  ***



Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on September 07, 2020, 02:29:42 PM
More:


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on September 07, 2020, 02:32:54 PM
The pics are coming out better today:


Hollywood, Baby Blue and Summer Breeze all ready to fire up and do radio service...


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: w8khk on September 07, 2020, 03:41:52 PM
Tom,

Did you see my posts referencing the links to the WB9ECK series-modulated rig?  (Reply #56 in this thread)  They should give you great confidence in your plans.

Your schematics look good.   Just a few comments....

I do not believe you are going to be able to set the carrier level using a diode string under the series modulator tubes.  Just ground the cathodes through the resistors,  The carrier set point will be a function of the amount of static voltage drop across the series modulators, and this is controlled by the DC bias component supplied to the 6LF6 screen grids via the MOSFET driver circuit.  Frank's design will likely include the components to be able to adjust the bias set point, and include the audio drive to the screens with the same circuit.  

I would also discuss  the T/R switching method with Frank, as you will probably be leaving the HVPS live during receive periods.  You show an isolated bias supply for the 4D32, and I assume that is cutoff bias, but you probably do not want the entire 2000 supply volts to appear across the 4D32 during receive. It may be best to have the series modulators also close to cutoff during receive, and have two resistors, one across the modulator, and one across the final, to share the 2000 volts, half across each part of the series circuit.  The resistor across the modulator will perform the NPL function, but the resistors will be large enough to be inconsequential with regard to dissipation, both in standby and transmit mode.  If the resistor across the series modulator is too low a resistance, you will not get close to the zero crossing on negative peaks.  It will also cause the 4D32 to endure the entire plate voltage supply during standby, probably not favorable.  Bottom line, you do not want excessive NPL current through that resistor.

I see you are metering screen current at the hot end of the screen dropping resistor.  If you do this, you might as well put the plate current meter up there too, and read each current individually.  An alternative is to leave the meter in the negative supply circuit as drawn, then move the screen current meter to the screen end of the dropping resistor to reduce the voltage on the meter assembly.  It would still need to be isolated, but the lower the exposed voltage, the better.

Let's see what Frank has in store for the driver and carrier set circuitry before proceeding further.


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on September 07, 2020, 04:08:40 PM
Bery, bery good, Rick!

I made all the meters HV isolated so that there is now a separate plate current reading. The screen is measured after the dropping resistor.  I got rid of the cathode diodes. Might as well make a plexiglass panel to mount and house the meters. I did just that with the PDM rig.

Yes, I believe Frank will have the audio driver modulator carrier idle and audio on the same lead into the screens.

The 1 to 2KV HV is keyed in Baby Blue via a step start and second relay. It drops off during receive.  All of my HV supplies in the shack do this. It was more of a safety issue to me than anything else. I hate that scary 60 Hz hum sound when in receive.

Yes, that's a floating grid bias fixed cutoff supply.  

I have no RF final stage keying cuz when the modulator is cut off with that 50K cathode relay/resistor, all current stops to the final.  At least my 6AQ5 did it like that.

I'll talk to Frank about the two resistors across the final and modulator. Sounds logical.

** So essentially, the only cap to ground in the 4D32 cathode/ grid circuit is the 500 pF 5KV, correct?  All other leads go to the 4D32 cathode....

I made the changes on the schematic and will leave the old one up there in case there are some more comments today - then change it tomorrow with the updated one.

You are making this too easy. I usually have to suffer more to get to this point... :-)

T


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: w8khk on September 07, 2020, 04:56:02 PM
Yes, the only cap to ground is the 500 pF, 5 KV bypass.  This will probably be small enough that it will not attenuate any highs, as you have a very low impedance source modulator. (I am not even sure that you need an RF choke between the cathode and the modulator plates, but it should not cause any problem either way.)  All other returns go back to the cathode, not ground, so you have true series modulation.  This is equivalent to high-level plate modulation as far as the final is concerned.

I believe the bypass capacitor from cathode to the 2.5 mH grid choke should go from cathode to the bottom of the grid leak resistor, not the junction between the choke and the resistor.  Your grid current meter must also be isolated, of course.

Since you are metering the plate voltage at the high side with an isolated meter, you might as well meter the screen current from the high side, and put the screen dropping resistor back where it was, keeping the potential on all the meter cases (except the grid current meter) all relative to the high voltage.  (I was looking a minimizing the HV on the isolated meters if you were going to measure plate voltage on the low side.)

I also prefer to switch and step-start my HV supplies for my AM rigs, but for my 4-400 linear on sideband, I leave the HV on and use cutoff bias during receive.  Too much relay clatter on sideband exchanges.....

Back to the resistors.... You will want the NPL resistor across the series modulator, but I would leave it out for the initial testing, then add it and compare performance under modulation.  Since you will kill the HV on receive, no need to shunt a resistor across the 4D32 final.  If the HV was always there, then it becomes an issue causing all the stress to Mr. Raytheon.

I wonder how fast your power supply bleeds down when going from transmit to receive?  If it is fast enough, then you probably do not even need a relay in the minus side of the series modulator.   If it does bleed down very slowly, then we really need to look at the overall picture to determine what components see the HV until it is gone.  But if you stay within the capability of the 4D32 to handle all the voltage, and have the NPL resistor across the sweepers, then there should be no issue.

I am not familiar with the grid circuits you have used earlier, for example the dual quads.  Did you intend to have an "EL" network for the grid drive?  The two .001 uF  (1000 pF) caps before and after the EL network just function as DC blockers, and have insufficient reactance to be active as part of the matching network.

You have me thinking of building one too!  No parts to buy, my junkbox is full (except for the LF6 toobs).  I wonder how it would drive my 8877 homebrew linear?  It has a 4000 volt 2 amp CCS power supply with variac. The plate transformer is over 250 pounds, from a Temco 5 KW S\shortwave broadcast transmitter. Filter is a 290 uF oil cap at 5 KV.  That certainly deserves a glitch resistor, ya?

Please keep me informed as you and Frank work on the input and DC set circuitry.  He does MAGIC with the sand state FETs.  I will be curious to see how he wraps the ribbons on it!





Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on September 07, 2020, 06:33:50 PM
OK on all, Rick.  I did the changes.

I am using the double grid DC blockers just to keep the HV off the RF input.  I will still float the input components as if they were hot in case, just to be cautious.

The HV supply dies slowly from a 200K bleeder. It works FB with the other rigs, so not worried.

This is a good start for today.  I hope you jump in and build one too.  That will undoubtedly accelerate our overall progress.  I will start looking around for parts and chassis materials.

Great driver for an 8877 linear. I have one here too that will be used from time to time. I also have a pair of 4X1s in linear and Mr. Ugly.  All need an AM driver that the FT-1000D could not fulfill.

The stand-alone 40 watt PW rig version shud be fun too.  I think we will be limited to 30-40 watts carrier due to the 6LF6 heat. But that's just FB for me cuz the peaks will be huge and clean. The NPL will be very important to be set up without splatter, but I know it can be done. I plan to put a stiff breeze on them. A muffin fan blowing air directly down, centered, is the best method I've found. The 4D32 will get some stray ventilation but will not really need much as a loafer.

T


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on September 07, 2020, 09:57:46 PM
Rick,

I forgot to comment on Bob/ECK's rig.  I contacted him a month ago about his rig. He said:    "As time went on and I was using my series mod rig I discovered some instability issues.  I added parasitic suppressors in a couple of different places."   And,    "If I had it to do over again Iíd use an 807W series modulating an 807W."

So I am prepared to add some parasitic suppressors.  I'm not sure about the 807W comment though.

Frank is making good headway with his MOSFET audio driver design.    He observed from the data sheet that at 0 volts on G1 and 160 on the screen it is hauling 1400 ma. He would be surprised if it needed more than 80 volts peak on the screens which is a good thang.  

Also, he found it necessary for the audio driver to have a negative supply. There is distortion at the negative peak without it.  So a +- supply will be required and the elimination of the diodes.
And he is working on adding negative feedback.  NFB will be the icing on the cake.

Looking back at some of my earlier schematics, I found that I had  .001 bypass caps from the top AND bottom of the grid leak resistor to ground. Is there any reason for me to pull out that top bypass on the rigs or is it OK?  I figgered that the 2.5 mH choke was enuff of impedance blocking.

Another thing I noticed on Bob/ECK's rig:  He used a resistor from the grid to the cathode for grid bias.  And then a blocking cap that fed the input RF circuitry. All of his input circuitry was at ground potential. Is that something we should be doing without a fixed cutoff bias supply?  I seem to remember that with the series modulator there was never any problems with damaging currents when the RF drive was taken off. This was on my 6AQ5 rig. It wud be so much simpler without floating the input RF circuitry.  Or, maybe the input L/C can already be connected to ground potential because it has an RF path to the cathode thru the 500 pF @ 5KV cap...

BTW, I found that I needed an 1 mH inductor and small capacitor between the plate of the series modulator and cathode of the RF tube due to instability. Maybe that is what Bob was referring to. I think it was more to keep the RF out of the audio. He said he didn't keep any documentation. This will all be determined in testing which can be a fun challenge.


T


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: w8khk on September 08, 2020, 02:24:07 AM
You bring up many different points here, so I am not sure I will hit all of them in one response.....

Bob mentioned instability, and an 807W.  The 807W has a form factor similar to the 6146, not so tall, shorter leads, less lead inductance, thus easier to tame parasitics.  His comments probably were not related as much to the modulator.  Typical bypassing and parasitic suppression techniques apply no matter what tube we are using.  I think the 4D32 will be fine so long as input and output circuits are isolated/shielded, top and bottom of chassis is probably sufficient.

Biasing and protection gets interesting, so many options.  Perhaps a landline discussion would save some time, I have so many irons in the fire right now.  But to start, we can use fixed and grid leak in combination, grid leak by itself, clamp tube, and another option is to have a method to detect that drive is available and just use the proper amount of grid leak bias, simplifying the circuit and build significantly. 

A simple diode detector that enables or disables the TR switching and HV if there is no drive would allow you to use just grid leak, tied back to just the 4D32 cathode.  The grid leak bias is produced by grid current through the leak resistor and a capacitor is preferred to retain that charge between RF cycles to bias the grid deep into Class-C for big positive modulation peaks.  RF chokes and bypass capacitors may be added for metering the grid current, but the DC path for generating the grid bias must be tied back to the cathode of the final, not the chassis ground.  The bias is developed across the grid leak resistor, and the capacitor in parallel with this resistor maintains the bias.  A clamp could use the same filament transformer for the 4D32, but I think there are simpler ways of protecting the final without resorting to a clamp tube.  Perhaps just sampling the grid current with a small relay would allow a fail-safe to prevent HV application in absence of RF drive.  I would lean toward eliminating the fixed bias supply, and simplifying the grid current metering wiring as much as possible.  I am not sure about the need for screen current metering either.  I was thinking a relay on the screen to ground the screen voltage to the cathode in the event no drive is present, you then protect the final in absence of drive, and you discharge the supply when switching to receive via the grounded screen dropping resistor.  But let's see what Frank suggests for T/R issues before we add redundant functions.

According to his schematic, Bob just had a grid leak resistor from grid to cathode, no capacitor.

The input RF circuit need not be floating.  The coupling capacitor from the grid tuning circuit will address the difference in DC and modulation, but have a low enough reactance at the operating frequency to pass the RF.  If you wish to protect the driver, I would suggest an RF choke in parallel to the input, like the safety choke at the pi net output, instead of another series capacitor.  The Input network can be an L, Pi, link coupled tank, whatever works for you, capacitively coupled to the final grid.  The 4D32 is a very stable tube, even without neutralization, but you must make sure that the additional wiring for the grid current metering is properly filtered and bypassed such that you do not provide an unwanted feedback link causing parasitics.  Metering is always desirable, but the circuitry sometimes "gets in the way."

The need for some negative bias on the modulator does not surprise me.  Bob's design used a negative bias on the 12AU7 cathode follower that drives the 6LF6 series modulator.  I indicated this in my original simplified modulator block diagram with the driver tube.

It's late, I will try to get back and have a look at progress tomorrow.


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on September 08, 2020, 02:45:54 PM
Hi Rick -

OK on all.

I updated the schematic based on all of our recent ideas. It is simplified and improved. I still need to make a decision about the RF drive failsafe if there is no fixed cut-off bias. A simple RF detected relay on the input may work. Your screen voltage idea seems interesting.

Having meters for all functions is important. I have many times tried to get away without a grid or screen meter but ended up adding one in later when there was no room... :-)   Hopefully these HV insulated meters will work without adding in lead length in the wrong places. It appears they will all have bypassing.  All meters will be safely mounted in an enclosed Plexiglass support.

WE saw Frank's MOSFET audio driver and it looks simple enuff with only two MOSFETS. He was able to get NFB included. It is comprised of the input and output stages of his most famous GFZ audio driver board except it is single ended for our purpose. Once finished maybe John/JSW can make a professional PC Board for it.   We need to decide on the audio drive needed for the 6LF6s to choose a critical resistor value and proper power supply level so it will probably be one of the last things we build.

Time now for slowly accumulating parts that are missing. I need to buy some alum sheet metal and RF tank parts. I'm really running low this time... ;)

T

** UPDATED SCHEMATIC BELOW on 9-8-2020 **

EDIT: Schematic correction: The screen meter "+" should go to the "+" side of the plate meter, directly to the B+.  Right now the plate meter reads the plate current and screen current combined.



Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: w9jsw on September 08, 2020, 05:32:36 PM
I'm your huckleberry....


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: KD1SH on September 09, 2020, 09:15:53 AM
Since this thread has been blessed with a notable lack of dumb questions, I feel I've just got to step in and fill that void. Why is the center tap of the 4D32's heater transformer tied to the cathode? The 4D32's heater is isolated from the cathode internally, so other than getting hot - as the heater should do - why does it need to get involved in the action at all? I've been trying to find specs on the 4D32's heater/cathode max voltage, but haven't seen it anywhere.


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on September 09, 2020, 11:23:44 AM
Since this thread has been blessed with a notable lack of dumb questions, I feel I've just got to step in and fill that void. Why is the center tap of the 4D32's heater transformer tied to the cathode? The 4D32's heater is isolated from the cathode internally, so other than getting hot - as the heater should do - why does it need to get involved in the action at all? I've been trying to find specs on the 4D32's heater/cathode max voltage, but haven't seen it anywhere.

Hi Bill,

The heater filament to cathode  voltage rating is usually only a few hundred volts on many tubes and by connecting the fil xfmr CT to the cathode of indirectly-heated tubes keeps the fil to cathode potential to near zero. It is not a problem with directly heated cathodes like a 3-500Z or other power grid tubes, but when a cathode depends on a separate heater, it is a good practice. In our case we are playing with 1500 volts or so that will be a swinging dick of destruction...

T


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: KD1SH on September 09, 2020, 12:03:20 PM
Ah, yes, the infamous S.D.D.  Figured that was the idea, but I wasn't sure exactly what the allowable cathode/heater potential was. I was thinking for some reason that it would be a lot higher. But then, the physical spacing between heater and cathode must necessarily be small. On the 807 it's only something like 130 volts. Now, of course, we shift the burden to the heater transformer; probably hard to find anything with isolation greater than 2500 V these days.

Since this thread has been blessed with a notable lack of dumb questions, I feel I've just got to step in and fill that void. Why is the center tap of the 4D32's heater transformer tied to the cathode? The 4D32's heater is isolated from the cathode internally, so other than getting hot - as the heater should do - why does it need to get involved in the action at all? I've been trying to find specs on the 4D32's heater/cathode max voltage, but haven't seen it anywhere.

Hi Bill,

The filament to cathode  voltage rating is usually only a few hundred volts on many tubes and by connecting the fil xfmr CT to the cathode of indirectly-heated tubes keeps the fil to cathode potential to near zero. It is not a problem with directly heated cathodes like a 3-500Z or other power grid tubes, but when a cathode depends on a separate heater, it is a good practice. In our case we are playing with 1500 volts or so that will be a swinging dick of destruction...

T


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on September 09, 2020, 02:06:04 PM
Ah, yes, the infamous S.D.D.  Figured that was the idea, but I wasn't sure exactly what the allowable cathode/heater potential was. I was thinking for some reason that it would be a lot higher. But then, the physical spacing between heater and cathode must necessarily be small. On the 807 it's only something like 130 volts. Now, of course, we shift the burden to the heater transformer; probably hard to find anything with isolation greater than 2500 V these days.


I've been in that boat of looking for a HV insulated filament xfmr many times.  A great solution is to take the core of a Variac and use the existing turns for the 120 V primary winding. Take off the arm leaving a donut hole to wind wire on.  Then wind  the appropriate number of HV insulated turns to come up with a 6.3V secondary with a center tap. Around 20 turns or so, depending on the exisiting primary turns should do it. Trial and error under load until you get 6.3 VAC.  The secondary wire can be HV insulated wire or regular wire wound onto a Kapston? or Mylar equivalent sheet that covers the 120V winding. I have also used a few layers of black electrical tape... or both the tape and insulated wire.  The wire needs to handle the fil current; in this case about 4A for the 4D32 fil.

I've done this for 4X1 PDM rigs and other applications that needed 10++ KV insulation.  Same goes for fixed grid voltage transformers that need to float due to series modulation.  And even screen supplies.  Series modulation thru the cathode means some extra work with meters, supplies and floating some parts at times.  But in the end that RF final acts just like a conventional class C final but without mod iron... predictable, efficient and easy to tune up.

T



Title: Re: PICTURES ~45 watt SERIES MOD homebrew Pissweaker rig
Post by: K1JJ on September 09, 2020, 09:55:12 PM
Planning and digging around for parts.

I like to do a little every day on various projects, even if just an hour each.  I was able to dig out some key parts from the cellar stash.

Yeah, I know, the vacuum cap is overkill, but that's all I had. It's 700 pF and 15 KV. It will be seeing 1500 - 2KV on peaks.  I found a nice roller inductor and a 2100 pF loading cap. I need to get two turns counters.

You will see the stripped down Variac ready for its 6.3V filament  HV insulated winding for the 4D32.  I also found a nice 10A 6.3V fil xfmr for the four 6LF6s which requires 8 Amps.

Gonna order some 1/8" aluminum sheet stock off eBay. Good deals there and cheap shipping. I want to make a chassis for this rig so I can hide the fil transformers and any power supply stuff underneath.   I am not sure if I will build a HV power supply into the rig yet. But with a 48" X 24" piece of aluminum sheet coming, I can build just about anything I want.

Would  like to fit the (newly designed for this rig) GFZ simplified MOSFET audio driver and +- power supply underneath too. The top level should have mainly RF stuff and tubes with viewing windows.  I'm not sure yet how to mount the four meters for HV protection, but it will be safe with a Plexiglas front cover so there is no way to touch the meter or hardware.

Hopefully by plowing into the project a little, I am apt to do more each day. It all adds up over time.  Not a bad start, huh?

T



Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on September 09, 2020, 09:57:58 PM
The parts need a good cleaning.   The 4D32 is a little too far away from the vacuum C1 plate capacitor for a short lead.   I want it in front for the viewing window. Must figger that out.

This used to be my workbench.  As you can see the rigs took over the workspace.  I have to roll in a work table when needed. The other side of the room has more tables with HB rigs and a 6' rack amplifier -  and the main operating position.  Everything works and each rig is ready to get on the air with a few switches...


Title: Re: PICTURES ~45 watt SERIES MOD homebrew Pissweaker rig
Post by: w8khk on September 09, 2020, 10:18:09 PM

Gonna order some 1/8" aluminum sheet stock off eBay. Good deals there and cheap shipping. I want to make a chassis for this rig so I can hide the fil transformers and any power supply stuff underneath.

Would like to see a link to the stock you are ordering.  I WISH I had a decent sheet metal shear and bending brake.  Then I would make my own chassis too.  What do you use for fabrication?  At home or contract out these steps?  Inquiring minds want to know!

For the plate tune and load, for this size project, I prefer the easy tuning with bread-slicers.  Don't even need a vernier drive.  Just some nice BIG knobs!  I guess I am just old school.

Can't wait to see Frank's circuit, especially the carrier level control he has designed.


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on September 09, 2020, 10:53:04 PM
Hi Rick,

There's lots of 1/8" aluminum sheet on e-Bay:

https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2047675.m570.l1313&_nkw=aluminum+sheet+.125&_sacat=0

Or other thicknesses. Some large pieces and some decent shipping prices.  There are copper sheets at ridiculously low prices if one were to use copper. (Like a Drake rig)   I just bought a 24" X 48" aluminum piece for $68.  


To cut the aluminum sheet stock (and Plexiglass) I use a 12" round blade table saw with a finer diamond blade.  The coarse blades are too dangerous. It is a scary operation either way. Wish I had a brake and shear.  To make a chassis I use 1"X 1" aluminum angle stock and 1/8" sheet, drill it and use 6-32 hardware to hold it together.  You can build anything like that including a panel / chassis connected by side struts.   I use hole saws on the drill press for old buzzard round 2.5" / 3" meters and various tube socket holes.  

If anyone has a 3000V ~400 pF breadslicer for sale I'd be interested. Hopefully smaller than what I am using now. I could use the vac cap as a backup for the other rigs.

T


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: Opcom on September 10, 2020, 03:25:44 PM
Looks great so far and I'm just getting my learning.. the trap shut and ears open thing on this. I have only 3 comments.

1.) the 6LF6s, why 4? not for the current..


I'm a 6CD6 man for the high current low voltage sweepers. It has only half the rated dissipation with a 20 Watt anode, but also only 6 bucks. It also handles high currents well.

If the whole series mod+RF circuit is running at 1200V with a 100V minimum drop across the 6LF6s and 550V across the chass C stage at carrier of 70W (all from the tube data), four 40W tubes gives a great margin. 160W dissipation and only 115W dissipated at carrier level. The bare minimum would be three 6LF6 or six 6CD6. I investigated how the plate dissipation changes over the modulation half cycle.

The heater is same voltage as the 6LF6, but uses 2.5A, vs. 2A for the 6LF6. I like to think that it means the cathode is larger, but the 6CD6GA is an older tube from 1954 compared to the 6LF6, which apparently began as the Philips/Amperex tube N30EL in 1966, and was registered as the 6LF6 and revised/improved until 1972. So there's 18 years of evolution between the two.

The datasheets present the class A plate current differently: as average for the 6CD6 and as peak for the 6LF6. The class A data is what seems to be given for sweep tubes to higlight the high current @ low voltage capability during the pulse operation.

design-center values JETEC
tube | Heater | (des ctr) | Cathode | Cathode | class A typical
type | W | Plate W | Avg. mA | Pk. mA | operation
6CD6GA | 15.75 | 20 | 200 | 700 | Eb=60,Eg2=100,Eg1=0,Ib=230
6LF6 | 12.6 | 40 | 176* | 800 | Eb=50,Eg2=175, Eg1=0,Ib=176


*6LF6 Data sheet gives only peak current values. Chart figure is based on datasheet's 22% duty cycle @ lowest class A plate voltage (22% of 800mA). Note that the 6CD6 data specifies the pulse duty as 15% (15% of 700 is 105mA) but does give the 200mA average value. The discrepancy is probably related to the 2x plate dissipation difference between the two tubes. The 6LF6 has a design maximum rating for cathode current of 500mA in an older Amperex data sheet, very impressive.

My inquisitiveness was related to how many 6LF6s are needed and why.

It was stated that four 6LF6 are probably excessive and that number were used for linearity enhancement purposes. I can only presume that linearity will be worked out with negative feedback and a solid state driver for the 6LF6s. A suspicion is that the screen voltage has much to do with sweep tube current, so the supply ought be well regulated for this topic's use as a linear DC amplifier, whereas it is not as important in the pulse operation as used in TV sets.

For series modulators, the required plate dissipation capacity of the modulator tubes can be calculated, assuming the DC load presented by the class C stage is constant (as it's usually assumed).
In the 4D32 datasheet telephony plate and screen modulation section, which should behave the same way whether the modulation is supplied from the positive (plate) side or the negative (cathode) side, the data is given:

Carrier:
Plate voltage 550V
Plate current 175mA
Carrier output 70W

crest of modulation cycle:
Plate voltage 1100V
plate current 350mA
peak output 280W

From this I used a spreadsheet to graph the dissipation of the series modulator. It's not the current rating but the plate dissipation rating where the 6LF6 comes out on top.


2.) Other tubes mentioned:
The 813 is supposedly good for suppressor modulation.
The 6550 appears in an ARRL handbook as making 20W on 20 meters but was said to be poor above that frequency band.

3.) The series modulation via the cathode+grids is "A-OK" by me but I've only ever tried it in series with the plate, that is, as the plate supply.
If it were supplied to the plate as in conventional plate modulation, then an old laboratory type variable regulated power supply could be modded and used to give a plate voltage swing from 0 to 500V or whatever voltage the supply was rated for at full current. Many were something like 0-500V @ 300mA. The advantage is a DC coupled amplifier already with feedback for good regulation. The main mod is to remove various bypass capacitors that would otherwise prevent the supply being modulated by an audio voltage. Removing all those caps sometimes causes instability, so the simpler the power supply the better. The voltage swing of a 500v supply is too small for this project LOL but is enough for a true 25W carrier transmitter. I did this a long time ago using a single 807 as the class C amp making about 5-6W carrier. The PS had two 6L6GCs as series tubes.

Not suggesting any changes, just remembering and thinking about it, figured out some things from the discussion.
I'm watching this as eagerly as everyone else.


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on September 10, 2020, 05:06:13 PM
Thanks for the analysis effort, Pat!

Let's see if I have this right and it agree roughly with your spreadsheet...   Say we are using 1500VDC as the main HV supply.   And let's say we have about 450 volts on the 4D32 at 120 mA  and 1000V across the 6LF6s in dead carrier state.  So 450V X .120 A = 54 watts input  X .75 efficiency = 40.5 watts RF carrier output.   At dead carrier  (the maximum dissipation point for the class A 6LF6s) there is 1000V X 120 mA = 120 watts dissipation.   Being 40 watt tubes X 4 equals 160 watts rated.  So pulling 120 watts from tubes rated at 160 watts is reasonable.  Even though they may not need it, I definitely plan on some air forced down from a quiet muffin fan.

Notice the rig is only putting out 40.5 watts. This is 34% efficiency for the modulator. (40.5w / 120w)        The 4D32 is running at less than 1/3  power capability in class C.   The bottleneck, if we want to call it that is the 6LF6s.

The current could be increased to 160 mA =  72 watts input to the 4D32 X .75 = 54 watts output. The 6LF6s would be singing for their supper at a full 160 watts of diss, but probably hang in there nicely.

Now as far as modulation %, with the  voltage going from 450V to 900V is 100% modulation.  What would 450V to 1500V produce?  Is this 200%+ ?  

I think four 6LF6s may be a good choice.  They will scratch out a cool 20 to 45 watts of carrier with huge positive peak headroom available for driving a big linear. (or barefoot)  A good working NPL will be needed to keep the bandwidth clean.  There are some rigs on the air running 150-200% audio that are reasonably clean. If the receiving station uses a sync detector we have a lot of possibilities.

I'm thinking that the 6LF6s will show their value when we start creeping up the HV to the 2KV area; with a heavily loaded 4D32 final and 40 watt carrier - seeing how far the peaks can be pushed cleanly.  I ran 2200V on my old Dual Quads 4D32 PDM rig, (series modulated)  so I know the 4D32 can take 2KV peak. Brutal, I know.   Had a lot of fun doing this with my little 5 watt 6AQ5 series modulated rig. Was seeing 200-300% audio when the carrier was backed down to 1 watt.  Again, we may find that in the real world of diode detectors we are limited to 125-150%, but it will be fun experimenting with this series modulated "big rig"...  ;D

T


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: w8khk on September 10, 2020, 06:29:34 PM
I would suggest planning your output tank values to allow optimum Q for a range of plate voltages on the 4D32.  With the roller and overkill plate tuning and loading caps, you probably already have that covered.  

The reason I suggest this is that you might find a higher voltage and somewhat lower current will play better with the series modulator.  Since it is not switch mode PDM at the modulator, you will not likely run the series tubes up to saturation without a hook in the audio peak waveform.  Instead of 450 volts at 120 mA,  I would suggest designing to allow up to 500 or 550 volts steady state at around 100 mA.  So long as your power supply provides enough voltage, and your sweep tubes can tolerate the highier cathode to plate voltage drop, you will handle the peaks cleaner, with somewhat less dissipation in the modulator.

Since this was intended to be a PW rig as a low end or driver transmitter, it seems appropriate to focus on plate voltages and modulation percentages that provide the cleanest signal before amplification.  I am not sure that positive peaks above 125 percent buy you as much as a good, dense, loud and clean average modulation percentage.  

When you start running audio through the modulator and look at the modulated waveform at on the plate voltage, it will become clear where the sweet spot is, and just how far up you can go on the peaks with cleanliness.  It is not necessary to consume all the excess dissipation available, let it coast and last a long time.  I was thinking of getting some of those 6LF6 bottles until I looked at how the audiophools have escalated the market price!


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on September 10, 2020, 08:52:30 PM
Yep, running  500-550VDC with less current sounds like a good idea. SummerBreeze, a single 4D32 rig, works very FB at 550 volts.  But it can get a mean streak and run 200 watts out fully modulated using 750V.   Of course, if only 20 watts RF is needed to drive a common smaller linear, then the HV can be moved lower.  Lots of flexibility with the roller inductor and lots of C1 and C2 to play with as well as a Variac on the HV.

I looked at the eBay 6LF6 tubes and they are higher than before. Back in 2012 I bought mine new for about $15 each.  And I thought that was high back then.   Notice I have them mounted on a plate that can be easily removed and replaced with something else if I blow them out. But it would be hard to replace such a low voltage, high current tube with one.

While I wait for my aluminum sheet, the choice to make a normal-sized rig like the last three or a smaller compact one with the HV and audio board/PS outboard is still open.  A total effort could be using an internal DDS VFO w/ 5 watt SS driver, HV supply and the GFZ board and PS all in one box.  That would become another 100 pound rig. Then, where do I put it?  A smaller, compact rig could sit on top of Baby Blue and borrow its HV and RF feedthru coax.   If I were really to scrounge, one side of the GFZ 813s  Hollywood MOSFET audio driver could power the series modulator, but that's getting real lazy.

T


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: KD6VXI on September 10, 2020, 10:00:43 PM
It's not the audiophools that drove the price up, it's the CBers.  The 6LF6 is the tube of choice for the sweep tube amps.

I remember seeing data sheets that showed 33 watts PDiss for the 6LF6.  That was probably the avg power dissipation.

Like I had said earlier, I've seen them run at 1100 to 1200 volts, 1 x 3 x 6 tubes.  Guys would see 2200 pep or more.  But the tubes sure didn't live long like that!

If you can find them, the 8950 compactron is pretty much the same tube, but 12 volt fil.  There is also the 20LF6 and the 30KD6 that will work, but have different voltage for the filament.  The 6KD6 is also the same tube damn near.  The 6HF5 is a hotter tube than the 6LF6 as well. As I remember, it's interchangeable.

Then their is the holy grail, the M2057.  That is an 8950 on steroids, designed to interchange with it, but made for rf amplification and not sweep tube use.  That tube has 40 some watts PDiss avg.  I once owned an amp with 10 of those as a final.

Hope that helps if you ever need to find replacements!

--Shane
KD6VXI


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: WD5JKO on September 11, 2020, 10:18:40 AM

The 7984 Compactron tube might be a candidate, and this one is still inexpensive!

A similar Compactron tube with a plate cap is the 8150.

PDF's attached.


Jim
Wd5JKO

 


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: KL7OF on September 11, 2020, 01:38:46 PM
Tom.....I am following with great interest..Could you provide details on how you change the power supply leads on the "same power supply, different deck" setups?  I want to use the same supply for multiple decks and I'm wanting a "quick change" pit stop type of connection(s)...Tnx....I like the cast mount on that Vac cap....


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on September 11, 2020, 02:44:30 PM
Tom.....I am following with great interest..Could you provide details on how you change the power supply leads on the "same power supply, different deck" setups?  I want to use the same supply for multiple decks and I'm wanting a "quick change" pit stop type of connection(s)...Tnx....I like the cast mount on that Vac cap....

Hi Steve,

You could simply tie the various rig's HV together and have HV on them all the time.  Or do what I did for my three big rigs... I use a single HV supply that generates 2500, 3500 and 4 KV via primary taps.  I put a big rotary bandswitch in the power supply and simply select the rig I want to get HV power to. Use a bandswitch of about the same size as you would for the tank of the biggest rig. IE, a 4KV supply should use a 4"-5" diameter bandswitch  assuming the contact spacing is adequate.   Also, you can get by with just one step start and one glitch resistor in the power supply as well as just one HV metering and breakers/ switches, etc. Lots of advantages to using just one big HV supply.

Yes, that cast alum vac cap mount is one of a kind. I got it from Dave / K2IJY 30 years ago and it has been in MANY recycled parts rigs since. And it lives again.

Jim, yes, those  7984/8150s looks like they should work. There are lots of choices and price should not stop someone from building this rig. Frank GFZ mentioned that there are similar versions of the 6LF6 tube running higher filaments for cheap prices on e-Bay.  Check out ebay for the 36LW6 ,  26LW6 and 6LW6. Where there's a will there's a way.

Rick, I was thinking about the grid meter for the 4D32.  Should I add a .001 5KV  cap from the bottom of the grid leak to ground?  I could connect the grid leak resistor directly to the tube pin and the other side (.001 to gnd) would be a short path for unwanted RF.   The  500 pF at the cathode to ground would complete the RF path to the cathode.   Watsa?

Shane, yep, those 6LF6s have taken center stage with the CB builders. We could say they are near the peak of tube engineering before money was stopped and then thrown at solid state devices for the next 60 years.  And here we are still appreciating them, just like an old muscle car or Harley.

T


Title: Re: ALUMINUM CHASSIS/ PANEL BUILT - Series modulated homebrew Pissweaker rig
Post by: K1JJ on September 19, 2020, 01:36:53 AM
You thought I forgot youse guys?  Nope.  The new series modulated 4D32 by four 6LF6s is now at the point of no return. I am definitely going to build it.  

Yesterday I received a 2' X 4' sheet of aluminum for $60 from eBay. Today there's a clean canvas awaiting a new life as a radio transmitter. All made from 1/8" aluminum including the chassis.

This may be the nicest chassis/ panel combination I ever made up. Rick was axing about how I build these, so here is a good example.  No welding, just 1"X 1" angle.  I usually recycle rigs but this is virgin aluminum.  You will notice there is room underneath for subterranean parts.   I decided to make the rig totally self contained with a HV supply, the new GFZ MOSFET audio driver designed for this rig, the +- 300V supply it needs as well as an internal DDS VFO and 5 watt driver for the 4D32.

A little secret... instead of using the table saw and getting wavy-gravy cuts, I traced out the nine cuts with a sharpie pen and brought the 2' X 4' piece down to the local metal shop. For only $20 they sheared the aluminum to perfection. Beautiful cuts! Well worth it. I might even have them cut the panel meter holes, viewing window and turns counter slots with their plasma cutter too. I'm getting too old for this sawing, cutting and filing stuff...  ;D

The four meters will be mounted behind the front panel using Plexiglass so they will be safe for the HV float.   Actually, when the bottom and top are added together, there is enough room for a party inside.

This is a desktop rig and the panel is 22" wide and 13" tall. The bottom chassis is 4" high.  I will build my own custom cabinet. The extra 3" (horizontal panel space) over a standard 19" rack is worth the custom work. There are lots of possibilities.  I have only one big 19" rack here and dislike them cuz they tend to hide the work later. I prefer to look at the insides often for a thrill. Table top is more my style for servicing too. Easy, slide-off cabinets I like.

**  Notice the parts lined up for selection and cleaning. I am running out of work space so will wheel in a table tmw.

** BTW, I decided to name this rig "Yaz" in honor of  Yaz 1, 2 ,3 and 4.  


T


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on September 19, 2020, 01:40:20 AM
BTW, I forgot to say... the chassis / panel combo is very strong!  It is of simple construction but those side struts really give it a boost.  The panel is rigid and there is no movement anywhere.  The alum angle technique works well.


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on September 19, 2020, 01:45:14 AM
The second photo, 31.jpg is a rare shot of Fabio,  the 4X1 rig built 10 years ago -  next to Hollywood, the 813 rig, built this year. They are the two big plate modulated AM rigs in the shack.


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on September 19, 2020, 01:54:38 AM
I plan to put big rubber feet on the bottom and leave the bottom open for ventilation.  It will probably be another heavy desktop rig, like 90 pounds or so.

I wondered today where I got all the parts.  For years I would go to flea markets with a camping knapsack on my back and filled it up throughout the day, dumped it off into the car and went back out.  I am getting seriously low on parts these days though.


Title: Re: The PERFECT C1 PLATE CAP - ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on September 20, 2020, 12:56:45 PM
Johnson 154-3   NOS  500pf, 2 kv variable capacitor

https://www.ebay.com/itm/500-pf-2-kv-variable-capacitor-EF-Johnson-154-3/114410331788?hash=item1aa3631e8c:g:qCoAAOSwwc5fXRaH

e-Bay from a ham - Here's the perfect breadslicer for this series modulated project:   a Johnson 500 pF at 2KV.  I plan to use 1200 to 1500V.    500 pF will cover 160M, no problem.  I've been looking for something like this instead of this big vac variable I was gonna use.  $55 and he has seven left in case you need one for your own project.    

This will also save me the cost of a turns counter and I'll be able to build the RF tank with shorter leads. Wat's not to like?

T


Title: Re: ALUMINUM CHASSIS/ PANEL BUILT - Series modulated homebrew Pissweaker rig
Post by: w8khk on September 21, 2020, 12:30:52 AM
Yesterday I received a 2' X 4' sheet of aluminum for $60 from eBay. Today there's a clean canvas awaiting a new life as a radio transmitter. All made from 1/8" aluminum including the chassis.

This may be the nicest chassis/ panel combination I ever made up. Rick was axing about how I build these, so here is a good example.  No welding, just 1"X 1" angle.  I usually recycle rigs but this is virgin aluminum.  You will notice there is room underneath for subterranean parts.   I decided to make the rig totally self contained with a HV supply, the new GFZ MOSFET audio driver designed for this rig, the +- 300V supply it needs as well as an internal DDS VFO and 5 watt driver for the 4D32.

A little secret... instead of using the table saw and getting wavy-gravy cuts, I traced out the nine cuts with a sharpie pen and brought the 2' X 4' piece down to the local metal shop. For only $20 they sheared the aluminum to perfection. Beautiful cuts! Well worth it. I might even have them cut the panel meter holes, viewing window and turns counter slots with their plasma cutter too. I'm getting too old for this sawing, cutting and filing stuff...  ;D


I really like the 1/8 inch thick aluminum approach.  The angle aluminum and stout sheet aluminum make for a very sturdy assembly.  I used to use the steel chassis that had "two bottoms".  No top surface, just a lip to be able to attach a top or bottom.  Recycle the chassis just with one flat sheet of aluminum.

I got hooked on making my own chassis when I was in the USAF at Davis Monthan in Tucson.  Befriended a ham that worked in the metal shop, who was responsible for custom-crafting skin and other components for the McDonnell Douglass F4C and D Phantom.  He offered the use of the shears and brakes, all hydraulically operated, as well as the drill presses and lathes.  Lots of scrap aluminum, big sheets, left over from other work, free for the taking.  All aircraft grade aluminum, so home-brewing in my barracks room was easier then than it is now!

I also like the fact that it will be self-contained with power supply and the VFO/driver circuits!

Tom, I like your move to a bread-slicer instead of the vac cap for the plate tuning.  Much easier and smaller, and quicker to find the dip with only 180 degrees rotation required.

Keep the updates and photos flowing!


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on September 21, 2020, 07:16:10 PM
Hi Rick -

FB on the access to the aluminum and shop in the military.  Hams usually take good care of each other once they find out.

I may start the drilling and blasting tmw. I'm just laying out the RF deck now.  Got the table in the room and ready to go.  It will be a slower going as I'm working on a few other things at the same time.

Hope you find the time to complete your 304TL X 3CX3000 series modulated rig this winter.

T

**  EDIT:  Schematic correction on page 3:  The screen meter "+" should go to the "+" side of the plate meter, directly to the B+.  Right now the plate meter reads the plate current and screen current combined.


Title: Re: Will Microwave Oven Transformer Work? ~25 watt homebrew Pissweaker rig
Post by: K1JJ on September 22, 2020, 09:58:03 PM
I need a power transformer for this project that will give me 1000-1200 VAC bridge for a 1500 VDC output under load.  It needs to be 10 pounds or less.   I am using a Variac, so anything will do that is 1000 to 1500V or so, about 200 mA, no CT.  

These microwave oven transformers for $25 are plentiful on e-Bay.  Has anyone used these and will they work for my application?  The common 700V TV transformers may arc if I use them as a bridge, but these microwave oven transformers seem like possible candidates.  I don't see any voltage ratings in these offerings.

I'd like to do it without using a voltage doubler or rewinding turns.  

** I see that one of the secondary legs is connected to the core. Can this be floated like a conventional transformer or is the insulation on this leg  poor and will have a breakdown?

Edit:     "Correct grounding is important with MOTs. The inner end of the secondary, which is near to the core, should be connected to the iron core. In many MOTs, this is already the case. The reason is that the insulation between core and winding is usually insufficient to withstand the full output voltage."   So this must be the downside.

T


Sample microwave transformer:

https://www.ebay.com/itm/Microwave-High-Voltage-Transformer-MD-903AMR-1-120V-60Hz/153948548777?_trkparms=aid%3D1110012%26algo%3DSPLICE.SOIPOST%26ao%3D1%26asc%3D20200420083544%26meid%3D8abfc6c972674086a0d859c0a9a68638%26pid%3D100008%26rk%3D3%26rkt%3D12%26sd%3D154032746913%26itm%3D153948548777%26pmt%3D1%26noa%3D0%26pg%3D2047675%26algv%3DPromotedSellersOtherItemsV2&_trksid=p2047675.c100008.m2219


Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K8DI on September 23, 2020, 12:16:19 PM
The other downside is the duty cycle. MOT's are rated (and used) a few minutes at a time. Note in pictures (or get one from a dead microwave) that the core is welded -- the laminations are sheets to reduce eddy current losses, the welds defeat the laminations being, well, laminations vs. a solid core.

Get one and put 120vac on the primary with the secondary open, for an hour, and measure the core temperature (I'm guessing you have an ir thermometer you can use, since they're also great for keeping track of tube seal and envelope temperatures).  My bet is the rise will be excessive...

I've scrapped a few microwaves out; unfortunately I've been to the recyclers between the last one and now or I'd just send you one to play with.

Ed



Title: Re: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig
Post by: K1JJ on September 23, 2020, 04:10:21 PM
OK, Ed, tnx for the info.

From what I read, the older MOTs back 15-20 years ago had a decent insulation and could be run with the two secondary leads above ground. But since then the manufacturers have cheapened out and engineered every possible savings, making them $5 cost with poor insulation to the core.    So maybe an older one might work.

I suppose a pair could be run with the gnd sides connected together.

I also read that the idle eddy currents are big as you said, so forget it.

All in all, I will look for a real transformer. I have one but it weighs 25 pounds - that would put too much weight into the rig making it past my 100 pound limit for moving around.

If anyone has a <10 pound HV transformer for sale, 200 mA,  1200-1500V AC, no CT, please let me know.   120VAC pri preferred.         (Or 1100-0-1100 CT or whatever)

T



Title: Re: UPDATE: Looking for ideas for a ~25 watt homebrew Pissweaker rig
Post by: K1JJ on October 05, 2020, 11:05:09 PM
Just want to post the latest progress on the  4D32 series modulated by quad 6LF6s...

Frank/ GFZ and John/ JSW are working on the MOSFET regulated +- 275VDC supply for the new GFZ audio driver board that will drive the 6LF6 screens. It's a nice design, pro PCBs and will eliminate the need for two chokes.  I am using an Antek toroidal xfmr, 230-0-230.

I am done with the drilling and blasting on the rig.  That homebrew 1/8" chassis and panel were a dream to work with. Big wide viewing window for the five tubes. I made a drilling mistake and made some deep scratches in the panel. Had to use a wee bit of body putty and sanded it out. Yes, I do body work on my rigs sometimes. I painted the front panel baby blue and will start mounting parts tmw.  I tried a few other colors like cream yellow, chrome, light and dark gray, but came back to baby blue again. That is such a nice color I just had to do it again.  Look back a few posts to see the last 3-500Z linear painted baby blue. Who can resist?

I will have all power supplies, including the HV and audio driver supply mounted inboard.  The only thing outboard now is the VFO. I am not sure if I will continue the tradition of driving this rig with the FT-1000D or not yet.   The HV transformer is a 1500-0-1500 20 pounder that will give me 0 to 2KV DC  using a Variac.  

I wound a 10KV insulated filament transformer for the 4D32 final using an old Variac core, Kapton insulated sheet and #12 wire.  Under actual tube filament load I adjusted the turns for 6.3 VAC.

I am using breadslicers for C1 and C2 and a roller inductor.  It is a heavy duty rig for only 40-50 watts output or so, but will probably be my cleanest of all.  I will probably drive the pair of 4X1s  linear amp with it when I get the urge.  Have a quiet muffin fan for the series modulators. The 4D32 will get a residual breeze, but is running way below its capacity.

I'll post some pictures when the parts are mounted.   I think it's gonna be a very FB rig and hoping others will build up the design later on after it is tested and working well.  With the abbreviated GFZ MOSFET audio board and series modulator, there are no audio bottlenecks.  PW or high power, it's "exquisite fidelity" all the way.     ;D

Stay tuned for pics soon.

T
AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands